drm/amd/amdgpu: Avoid writing GMC registers under sriov in gmc9

[Why]
For Vega10, disabling gart of gfxhub could mess up KIQ and PSP
under sriov mode, and lead to DMAR on host side.

[How]
Skip writing GMC registers under sriov.

Signed-off-by: YuBiao Wang <YuBiao.Wang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>