drm/amdgpu: Fix some unload driver issues

If have memory leak, maybe it will have issue in
ttm_bo_force_list_clean-> ttm_mem_evict_first.

Set adev->gart.ptr to null to avoid to call
amdgpu_gmc_set_pte_pde to cause ptr issue pointer when
calling amdgpu_gart_unbind in amdgpu_bo_fini which is after gart_fini.
Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_gart.c

@@ -202,6 +202,7 @@ void amdgpu_gart_table_vram_free(struct amdgpu_device *adev)
 		return;
 	}
 	amdgpu_bo_unref(&adev->gart.bo);
+	adev->gart.ptr = NULL;
 }
 
 /*