drm: fix warning on alpha

From: Andrew Morton <akpm@osdl.org>

On alpha:

drivers/char/drm/via_dmablit.h:44: error: field `direction' has incomplete type

Cc: Dave Airlie <airlied@linux.ie>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/char/drm/via_dmablit.h

@@ -30,6 +30,8 @@
 #ifndef _VIA_DMABLIT_H
 #define _VIA_DMABLIT_H
 
+#include <linux/dma-mapping.h>
+
 #define VIA_NUM_BLIT_ENGINES 2
 #define VIA_NUM_BLIT_SLOTS 8