drm/amd/display: Don't allow pstate if no support in blank

[Why]
We will hang if we report switch in VACTIVE but not in VBLANK and DPG_EN = 1

[How]
Block switch in ACTIVE if not supported in BLANK

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>