drm/amdgpu/vcn4.0.3: read back register after written

The addition of register read-back in VCN v4.0.3 is intended to prevent
potential race conditions.

Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Signed-off-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>