drm/amdgpu/vcn: Fix the register setting for vcn1

Correct the code error for setting register UVD_GFX10_ADDR_CONFIG.
Need to use inst_idx, or it only will set VCN0.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>