[media] video: Kconfig: Select VIDEOBUF2_DMA_CONTIG for VIDEO_MX2

Fix the following build error:

LD      .tmp_vmlinux1
drivers/built-in.o: In function `mx2_camera_init_videobuf':
clkdev.c:(.text+0xcfaf4): undefined reference to `vb2_dma_contig_memops'
drivers/built-in.o: In function `mx2_camera_probe':
clkdev.c:(.devinit.text+0x5734): undefined reference to `vb2_dma_contig_init_ctx'
clkdev.c:(.devinit.text+0x5778): undefined reference to `vb2_dma_contig_cleanup_ctx'
drivers/built-in.o: In function `mx2_camera_remove':
clkdev.c:(.devexit.text+0x89c): undefined reference to `vb2_dma_contig_cleanup_ctx'

commit c6a41e32 ([media] media i.MX27 camera: migrate driver to videobuf2)
missed to select VIDEOBUF2_DMA_CONTIG in Kconfig.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>