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>