drm/amdgpu/vcn: Need to unpause dpg before stop dpg

Need to unpause dpg first, or it will hit follow error during stop dpg:
"[drm] Register(1) [regUVD_POWER_STATUS] failed to reach value 0x00000001 != 0x00000000n"

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>