usb: gadget: uvc: Add support for DMABUF importing

Activate the videobuf2 DMABUF support. As vb2-vmalloc supports the
importer role only, exporting buffers isn't supported yet. When the
exporter role will be implemented in vb2-vmalloc the UVC gadget driver
will automatically gain support for it.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>