drm/amd/display: Check engine is not NULL before acquiring

[Why]
Engine can be NULL in some cases, so we must not acquire it.

[How]
Check for NULL engine before acquiring.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>