drm/amdgpu/psp: Correct and refine the vmr support. (v2)

Currently driver only psp v11 support vmr.

v2: squash in unused variable removal (Alex)

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>