drm/amdgpu: allow huge invalid mappings on GMC8

Only GMC9 supports true huge pages, but we can still free invalid mappings
on GMC8.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>