drm/qxl: handle NULL bo->resource in move callback

The ttm bo now initially has NULL bo->resource, and leaves the driver
the handle that. However it looks like we forgot to handle that for qxl.
It looks like this will just null-ptr-deref in qxl_bo_move(), if
bo->resource is NULL.

Fix this by calling move_null() if the new resource is TTM_PL_SYSTEM,
otherwise do the multi-hop sequence to ensure can safely call into
ttm_bo_move_memcpy(), since it might also need to clear the memory.
This should give the same behaviour as before.

Fixes: 180253782038 ("drm/ttm: stop allocating dummy resources during BO creation")
Signed-off-by: Matthew Auld <[e-mail]>
Cc: Christian König <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Acked-by: Christian König <[e-mail]>
Signed-off-by: Christian König <[e-mail]>

drivers/gpu/drm/qxl/qxl_ttm.c

···
 	struct ttm_resource *old_mem = bo->resource;
 	int ret;
 
+	if (!old_mem) {
+		if (new_mem->mem_type != TTM_PL_SYSTEM) {
+			hop->mem_type = TTM_PL_SYSTEM;
+			hop->flags = TTM_PL_FLAG_TEMPORARY;
+			return -EMULTIHOP;
+		}
+
+		ttm_bo_move_null(bo, new_mem);
+		return 0;
+	}
+
 	qxl_bo_move_notify(bo, new_mem);
 
 	ret = ttm_bo_wait_ctx(bo, ctx);