drm/amd/amdgpu: enable PG_EN bit in powergating UVD

Enable the PG_EN bit just before the SMU would be tasked
with the PG transition.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>