drm/amd/display: If P-State is supported try SubVP for smaller vlevel

drm/amd/display: If P-State is supported try SubVP for smaller vlevel

[Description]
- To reduce vlevel further, we can try to apply subvp on
  configs that already support p-state since the natural
  p-state support may not allow for DPM0.
- Add code to try subvp to reduce UCLK DPM level further
  if already supported, but don't use subvp if it does not
  optimize the DPM level even lower

Reviewed-by: Samson Tam <samson.tam@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>