[media] tw686x: Specify that the DMA is 32 bits

Set vb2_queue.gfp_flags to GFP_DMA32. Otherwise it will start to
create bounce buffers which is something you want to avoid since
those are in limited supply.

Without this patch, DMA scatter-gather may not work because
machines can ran out of buffers easily.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>