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.
Fixes: effbf6ca7eafda ("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>