media: staging/imx: always select VIDEOBUF2_DMA_CONTIG

I ran into a rare build error during randconfig testing:

drivers/staging/media/imx/imx-media-capture.o: In function `capture_stop_streaming':
imx-media-capture.c:(.text+0x224): undefined reference to `vb2_buffer_done'
drivers/staging/media/imx/imx-media-capture.o: In function `imx_media_capture_device_register':
imx-media-capture.c:(.text+0xe60): undefined reference to `vb2_queue_init'
imx-media-capture.c:(.text+0xfa0): undefined reference to `vb2_dma_contig_memops'

While VIDEOBUF2_DMA_CONTIG was already selected by the camera driver,
it wasn't necessarily there with just the base driver enabled.
This moves the 'select' statement to the top-level option to make
sure it's always available.

Fixes: 64b5a49d ("[media] media: imx: Add Capture Device Interface")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>