drm/amdgpu: cleanup unnecessary virt sriov check in amdgpu attribute

the amdgpu device attribute node will be created accordding to sriov vf
mode at runtime.
cleanup unnecessary sriov check in attribute operation function.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>