[media] s5p_mfc: remove a dead code

As reported by smatch:
	drivers/media/platform/s5p-mfc/s5p_mfc.c:1340 s5p_mfc_runtime_resume() warn: this array is probably non-NULL. 'm_dev->alloc_ctx'

alloc_ctx can never be NULL, as it is embeeded inside the struct
s5p_mfc_dev.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>