drm/amdgpu/vcn:fix dpg pause mode hang issue

Use mmUVD_SCRATCH2 tracking decode write point.
It will help avoid dpg pause mode hang issue.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>