drm/amd/amdgpu: add mutex locking for both DPM and PP based powergating for UVD/VCE

This adds a mutex lock for both DPM/PP around the changes in
power gating state so that userspace can poll registers without
a race condition on power state.

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>