drm/amd/display: Wait DMCUB to idle state before reset.

[WHY]
Very low rate to cause memory access issue while resetting
DMCUB after the halt command was sent to it.
The process of stopping fw of DMCUB may be timeout, that means
it is not in idle state, such as the window frames may still be
kept in cache, so reset by force will cause MMHUB hang.

[HOW]
After the halt command was sent, keep checking the DMCUB state until
it is idle.

Reviewed-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: hengzhou <Hengyong.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>