drm: drm_pci needs dma-mapping.h

On alpha:

WARNING: "dma_free_coherent" [drivers/char/drm/drm.ko] undefined!
WARNING: "dma_alloc_coherent" [drivers/char/drm/drm.ko] undefined!
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/char/drm/drm_pci.c

@@ -37,6 +37,7 @@
  */
 
 #include <linux/pci.h>
+#include <linux/dma-mapping.h>
 #include "drmP.h"
 
 /**********************************************************************/