[media] media-entity.c: get rid of var length arrays

Fix those sparse warnings:
	drivers/media/media-entity.c:238:17: warning: Variable length array is used.
	drivers/media/media-entity.c:239:17: warning: Variable length array is used.

That allows sparse and other code check tools to verify if the
function is using more stack than allowed.

It also solves a bad Kernel pratice of using var length arrays
at the stack.

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