drm/amd/display: fix null pointer access in gpu reset

drm/amd/display: fix null pointer access in gpu reset

During GPU reset, when receiving a DMCUB OUTBUX0 interrupt,
DAL code will set it to be OUTBOX interrupt and sets hw interrupt.
However, OUTBOX interrupt is not registered yet, so a NULL pointer
access will be executed.

Call Trace:
  dal_irq_service_set+0x30/0x90 [amdgpu]
  dc_interrupt_set+0x24/0x30 [amdgpu]
  amdgpu_dm_set_dmub_outbox_irq_state+0x22/0x30 [amdgpu]
  amdgpu_irq_update+0x77/0xa0 [amdgpu]
  amdgpu_irq_gpu_reset_resume_helper+0x67/0xa0 [amdgpu]
  amdgpu_do_asic_reset+0x219/0x260 [amdgpu]
  amdgpu_device_gpu_recover.cold+0x8c5/0xb64 [amdgpu]
  amdgpu_debugfs_gpu_recover_show+0x2c/0x60 [amdgpu]
  seq_read_iter+0xc2/0x450
  ? do_anonymous_page+0x22c/0x3b0
  seq_read+0xf9/0x140
  full_proxy_read+0x5c/0x90
  vfs_read+0xaa/0x190
  ksys_read+0x67/0xe0
  __x64_sys_read+0x1a/0x20

Fixes: d55deb29271cd9 ("drm/amdgpu/display: remove an old DCN3 guard")
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-and-tested-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>