drm/amd/display: Query DMCUB for dp alt status

[Why]
To avoid hanging RDPCSPIPE when INTERCEPTB isn't set.

DMCUB owns control of that bit so DMCUB should manage returning the
information driver needs for link encoder control.

[How]
Add a new DMCUB command to return dp alt disable and dp4 information.

Reviewed-by: Hansen Dsouza <hansen.dsouza@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>