drm/mediatek: include dma-mapping header

Although it builds fine here in my arm cross compile, it seems
either via some other patches in -next or some Kconfig combination,
this fails to build for everyone.

Include linux/dma-mapping.h should fix it.
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/mediatek/mtk_drm_drv.c

@@ -17,6 +17,7 @@
 #include <linux/of_address.h>
 #include <linux/of_platform.h>
 #include <linux/pm_runtime.h>
+#include <linux/dma-mapping.h>
 
 #include "mtk_drm_crtc.h"
 #include "mtk_drm_ddp.h"