drm/amdgpu: fix the gart table cleared issue for S3

Something writes over the first 8 MB so reserve this
on vega10 until we root cause it.

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