[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>

drivers/media/platform/s5p-jpeg/Makefile

 s5p-jpeg-objs := jpeg-core.o
-obj-$(CONFIG_VIDEO_SAMSUNG_S5P_JPEG) := s5p-jpeg.o
+obj-$(CONFIG_VIDEO_SAMSUNG_S5P_JPEG) += s5p-jpeg.o

drivers/media/platform/s5p-mfc/Makefile

-obj-$(CONFIG_VIDEO_SAMSUNG_S5P_MFC) := s5p-mfc.o
+obj-$(CONFIG_VIDEO_SAMSUNG_S5P_MFC) += s5p-mfc.o
 s5p-mfc-y += s5p_mfc.o s5p_mfc_intr.o
 s5p-mfc-y += s5p_mfc_dec.o s5p_mfc_enc.o
 s5p-mfc-y += s5p_mfc_ctrl.o s5p_mfc_pm.o