drm/amd/display: Switch DMCUB notify idle command to NO_WAIT

[Why]
Race condition between notification of driver idle and the command being
processed. We could theoretically enter idle between the submission and
the wait for idle that occurs after.

[How]
Switch the notification to NO_WAIT to avoid the RPTR access.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Sung joon Kim <sungjoon.kim@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>