drm/amdgpu: fix s3 resume back, uvd dpm randomly can't disable.

the value of last_mclk_dpm_enable_mask will be changed if
other clients(vce,dal) trigger set power state between enable
and disable uvd dpm.

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