[media] omap3isp: stat: Share common code for buffer allocation

Move code common between the isp_stat_bufs_alloc_dma() and
isp_stat_bufs_alloc_iommu() functions to isp_stat_bufs_alloc().

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>