drm/amd/display: Wait for mailbox ready when powering up DMCUB

[Why]
Otherwise we can send commands too early and they don't execute until
the next command is sent.

[How]
Check the extra status bit when polling for HW powered up.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>