drm/i915/dp_mst: Fix mst_mgr lookup during atomic check

drm/i915/dp_mst: Fix mst_mgr lookup during atomic check

If an MST connector was disabled in the old state during a commit, the
connector's best_encoder will be NULL, so we can't look up mst_mgr via
it. Do the lookup instead via intel_connector->mst_port which always
points to the primary encoder.

This fixes the following:
[   58.922866] BUG: kernel NULL pointer dereference, address: 0000000000000170
[   58.922867] #PF: supervisor read access in kernel mode
[   58.922868] #PF: error_code(0x0000) - not-present page
[   58.922869] PGD 0 P4D 0
[   58.922870] Oops: 0000 [#1] PREEMPT SMP NOPTI
[   58.922872] CPU: 0 PID: 133 Comm: kworker/0:2 Tainted: G     U             6.0.0-rc3-imre+ #560
[   58.922874] Hardware name: Intel Corporation Alder Lake Client Platform/AlderLake-P DDR5 RVP, BIOS ADLPFWI1.R00.3135.A00.2203251419 03/25/2022
[   58.922874] Workqueue: events output_poll_execute [drm_kms_helper]
[   58.922879] RIP: 0010:intel_dp_mst_atomic_check+0xbb/0x1c0 [i915]
[   58.922955] Code: 5b 7b f6 ff 84 c0 75 41 48 8b 44 24 18 65 48 2b 04 25 28 00 00 00 0f 85 ff 00 00 00 48 8b 45 10 48 8b 93 10 07 00 00 4c 89 e7 <48> 8b b0 70 01 00 00 48 83 c4 20 5b 5d 48 81 c6 f0 0c 00 00 41 5c
[   58.922956] RSP: 0018:ffffc90000633a88 EFLAGS: 00010246
[   58.922957] RAX: 0000000000000000 RBX: ffff888117d19000 RCX: ffff888101893308
[   58.922958] RDX: ffff888122981000 RSI: ffffffff82309ecc RDI: ffff888114da6800
[   58.922959] RBP: ffff8881094bab48 R08: 0000000081917436 R09: 0000000068191743
[   58.922960] R10: 0000000000000001 R11: 0000000000000001 R12: ffff888114da6800
[   58.922960] R13: ffff8881143f8000 R14: 0000000000000000 R15: ffff888119bf2000
[   58.922961] FS:  0000000000000000(0000) GS:ffff888496200000(0000) knlGS:0000000000000000
[   58.922962] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   58.922962] CR2: 0000000000000170 CR3: 0000000005612004 CR4: 0000000000770ef0
[   58.922963] PKRU: 55555554
[   58.922963] Call Trace:
[   58.922964]  <TASK>
[   58.922966]  drm_atomic_helper_check_modeset+0x3f8/0xc70 [drm_kms_helper]
[   58.922972]  intel_atomic_check+0xb1/0x3180 [i915]
[   58.923059]  ? find_held_lock+0x2b/0x80
[   58.923064]  drm_atomic_check_only+0x5d3/0xa60 [drm]
[   58.923082]  drm_atomic_commit+0x56/0xc0 [drm]
[   58.923097]  ? drm_plane_get_damage_clips.cold+0x1c/0x1c [drm]
[   58.923114]  drm_client_modeset_commit_atomic+0x235/0x280 [drm]
[   58.923132]  drm_client_modeset_commit_locked+0x5b/0x190 [drm]
[   58.923148]  drm_client_modeset_commit+0x24/0x50 [drm]
[   58.923164]  drm_fb_helper_set_par+0xae/0xe0 [drm_kms_helper]
[   58.923171]  drm_fb_helper_hotplug_event+0xd5/0xf0 [drm_kms_helper]
[   58.923178]  output_poll_execute+0xac/0x200 [drm_kms_helper]
[   58.923187]  process_one_work+0x268/0x580
[   58.923190]  ? process_one_work+0x580/0x580
[   58.923191]  worker_thread+0x52/0x3b0
[   58.923193]  ? process_one_work+0x580/0x580
[   58.923195]  kthread+0xf0/0x120
[   58.923196]  ? kthread_complete_and_exit+0x20/0x20
[   58.923198]  ret_from_fork+0x1f/0x30
[   58.923202]  </TASK>

Fixes: 6268101f2da1 ("drm/display/dp_mst: Don't open code modeset checks for releasing time slots")
Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220901161933.1004778-1-imre.deak@intel.com