drm/amdgpu/vg20:Restruct uvd to support multiple uvds

Vega20 has dual-UVD. Need Restruct amdgpu_device::uvd to support
multiple uvds. There are no any logical changes here.

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>