[media] s5p-mfc: Simplify alloc/release private buffer functions

Change parameters for s5p_mfc_alloc_priv_buf() and s5p_mfc_release_priv_buf()
functions. Instead of DMA device pointer and a base, provide common MFC
device structure and memory bank context identifier.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Tested-by: Smitha T Murthy <smitha.t@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>