[media] s5p-mfc: Allocate firmware with internal private buffer alloc function

Once firmware buffer has been converted to use s5p_mfc_priv_buf structure,
it is possible to allocate it with existing s5p_mfc_alloc_priv_buf()
function. This change will help to reduce code variants in the next
patches.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-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>