[media] uvcvideo: Use videobuf2 .get_unmapped_area() implementation

The get_unmapped_area() operation was forgotten during conversion to
videobuf2. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>