drm/amdgpu: Skip access GRBM_CNTL under SRIOV on gfx_v11

[Why]
GRBM_CNTL is a PF_only register on gfx_v11.
RLCG interface will return "out of range" under SRIOV VF.

[How]
Skip access GRBM_CNTL under gfx_v11 SRIOV VF.

Signed-off-by: Yifan Zha <Yifan.Zha@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>