staging: drm/omap: Fix usage of IS_ERR_OR_NULL and PTR_ERR

Return PTR_ERR(-ENOMEM) if dmm_txn_init cannot allocate a
refill engine.
Signed-off-by: Andy Gross <andy.gross@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/omapdrm/omap_dmm_tiler.c

@@ -296,7 +296,7 @@ static int fill(struct tcm_area *area, struct page **pages,
 
 	txn = dmm_txn_init(omap_dmm, area->tcm);
 	if (IS_ERR_OR_NULL(txn))
-		return PTR_ERR(txn);
+		return PTR_ERR(-ENOMEM);
 
 	tcm_for_each_slice(slice, *area, area_s) {
 		struct pat_area p_area = {