drm/amdgpu: drop the backing store when DMA-buf imports are evicted

Instead of moving this to the SYSTEM domain just drop the backing store
and let the resulting allocation be freed.
Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -204,6 +204,12 @@ static void amdgpu_evict_flags(struct ttm_buffer_object *bo,
 		.flags = TTM_PL_MASK_CACHING | TTM_PL_FLAG_SYSTEM
 	};
 
+	if (bo->type == ttm_bo_type_sg) {
+		placement->num_placement = 0;
+		placement->num_busy_placement = 0;
+		return;
+	}
+
 	if (!amdgpu_ttm_bo_is_amdgpu_bo(bo)) {
 		placement->placement = &placements;
 		placement->busy_placement = &placements;