drm/amdgpu: refine uvd4.2 init/stop code.

1. set uvd_status busy before uvd_start.
2. clear uvd_status to 0 after uvd stop.
   smu firmware may check uvd_status.
3. wait uvd idle before stop uvd.
4. not start uvd when hw_init.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>