drm/amd/display: For no plane case set pstate support in validation

- Previously update_clocks was overriding pstate support if
  it checked that there were no planes
- However, P-State support should be determined in validation
  phase instead
- This fixes an issue where a transition from FPO -> no planes
  expects UCLK MAX, but update_clocks was overriding to set
  UCLK to min

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>