[media] omap3isp: queue: Merge the prepare and sglist functions

In preparation for the switch to the DMA API merge the two functions
that handle buffer preparation for the USERPTR cases (both page-backed
and non page-backed memory).

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>