drm/amd/display: Add p-state debugging

[WHY]
P-State related issues are fairly common but currently
there is no way to debug these issues after the fact.

[HOW]
Add helpful registers to HW state queries

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Sung Lee <sunglee@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>