drm/amdgpu: Skip access PF-only registers on gfx10/gfxhub2_1 under SRIOV

[Why]
RLCG interface returns "out-of-range" error under SRIOV VF when accessing
PF-only registers.

[How]
Skip access PF-only registers on gfx10/gfxhub2_1 under SRIOV.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: ZhenGuo Yin <zhenguo.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>