drm/amdgpu: add missing cleanups for Polaris12 UVD/VCE on suspend

Perform proper cleanups on UVD/VCE suspend: powergate enablement,
clockgating enablement and dpm disablement. This can fix some hangs
observed on suspending when UVD/VCE still using(e.g. issue
"pm-suspend" when video is still playing).

Signed-off-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>