media: staging: media: imx: imx7-media-csi: Decouple from imx_media_dma_buf

Decouple from the imx_media_dma_buf structure defined in shared helpers
by duplicating it in the imx7-media-csi driver, along with the two small
alloc and free functions. No functional change intended.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>