drm/amdgpu: sriov restrict max_pfn below AMDGPU_GMC_HOLE

sriov need to restrict max_pfn below AMDGPU_GMC_HOLE.
access the hole results in a range fault interrupt IIRC.

Signed-off-by: Wentao Lou <Wentao.Lou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>