drm: fix compile failure by including <linux/swiotlb.h>

On tile architecture (with "make allyesconfig") including
<linux/swiotlb.h> is required to call swiotlb_nr_tbl().
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -28,6 +28,7 @@
  */
 
 #include <core/engine.h>
+#include <linux/swiotlb.h>
 
 #include <subdev/fb.h>
 #include <subdev/vm.h>

drivers/gpu/drm/radeon/radeon_ttm.c

@@ -38,6 +38,7 @@
 #include <drm/radeon_drm.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
+#include <linux/swiotlb.h>
 #include "radeon_reg.h"
 #include "radeon.h"