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 <thierry.reding@gmail.com>
CC: Mikko Perttunen <mperttunen@nvidia.com>
CC: dri-devel@lists.freedesktop.org
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>