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>