drm/amdgpu/vg20:Enable the 2nd instance for uvd

For Vega20, set num of uvd instance to 2, to enble 2nd instance.
The IB test build-in registers need update for vega20 2nd instance.

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