[media] s5p-mfc: Replace mem_dev_* entries with an array

Internal MFC driver device structure contains two pointers to devices used
for DMA memory allocation: mem_dev_l and mem_dev_r. Replace them with the
mem_dev[] array and use defines for accessing particular banks. This will
help to simplify code in the next patches.

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>