drm/v3d: Set the correct DMA mask according to the MMU's limits.

On 7278, we've got 40 bits to work with.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190419001014.23579-2-eric@anholt.net


Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>