drm/amdgpu: Enable VCN static PG by default on RV

Enable static VCN powergating by default on Raven.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>