[media] s5p makefiles: don't override other selections on obj-[ym]

The $obj-m/$obj-y vars should be adding new modules to build, not
overriding it. So, it should never use
	$obj-y := foo.o
instead, it should use:
	$obj-y += foo.o

Failing to do that is very bad, as it will suppress needed modules.

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