[media] s5p_mfc: Remove redundant casts

Both sides of these assignments actually have type "const struct
vb2_mem_ops *", so the casts are unnecessary and slightly confusing.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>