drm/i915: Do not create a new max_bpc prop for MST connectors

drm/i915: Do not create a new max_bpc prop for MST connectors

We're not allowed to create new properties after device registration
so for MST connectors we need to either create the max_bpc property
earlier, or we reuse one we already have. Let's do the latter apporach
since the corresponding SST connector already has the prop and its
min/max are correct also for the MST connector.

The problem was highlighted by commit 7bf14b3bbff5 ("drm/kms:
Catch mode_object lifetime errors") which results in the following
spew:
[ 1330.878941] WARNING: CPU: 2 PID: 1554 at drivers/gpu/drm/drm_mode_object.c:45 __drm_mode_object_add+0xa0/0xb0 [drm]
...
[ 1330.879008] Call Trace:
[ 1330.879023]  drm_property_create+0xba/0x180 [drm]
[ 1330.879036]  drm_property_create_range+0x15/0x30 [drm]
[ 1330.879048]  drm_connector_attach_max_bpc_property+0x62/0x80 [drm]
[ 1330.879086]  intel_dp_add_mst_connector+0x11f/0x140 [i915]
[ 1330.879094]  drm_dp_add_port.isra.20+0x20b/0x440 [drm_kms_helper]
...

Cc: stable@vger.kernel.org
Cc: Lyude Paul <lyude@redhat.com>
Cc: sunpeng.li@amd.com
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <sean@poorly.run>
Fixes: fe9616dc47fc ("drm/i915: Add max_bpc property for DP MST")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820161657.9658-1-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>