gpu: host1x: Add missing DMA API include

Host1x seems to be relying on picking up dma-mapping.h transitively from
iova.h, which has no reason to include it in the first place. Fix the
former issue before we totally break things by fixing the latter one.

CC: Thierry Reding <[e-mail]>
CC: Mikko Perttunen <[e-mail]>
CC: [e-mail]
Signed-off-by: Robin Murphy <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>

drivers/gpu/host1x/bus.c

···
  */
 
 #include <linux/debugfs.h>
+#include <linux/dma-mapping.h>
 #include <linux/host1x.h>
 #include <linux/of.h>
 #include <linux/seq_file.h>