drm/amd/display: Set WM set A to 0 if full pstate not supported

[Why]
If full pstate is not supported, we should set WM set A
to 0 to prevent any hangs

[How]
If pstate is not supported, set watermark set A to 0

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>