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 <[e-mail]>
Acked-by: Maarten Lankhorst <[e-mail]>

drivers/gpu/drm/nouveau/nouveau_bo.c

···
  */
 
 #include <core/engine.h>
+#include <linux/swiotlb.h>
 
 #include <subdev/fb.h>
 #include <subdev/vm.h>

drivers/gpu/drm/radeon/radeon_ttm.c

···
 #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"