CHROMIUM: v4l2-mem2mem: Implement dmabuf mmap for videobuf2-dma-contig

CHROMIUM: v4l2-mem2mem: Implement dmabuf mmap for videobuf2-dma-contig

videobuf2-dma-contig buffers already allow DMABUF exporting; now hook up
the mmap functions for these DMABUFs.

Signed-off-by: John Sheu <sheu@google.com>
BUG=chromium-os:38376
BUG=chromium:167417
TEST=local build, run on snow
Change-Id: Ia415c037b1fe9d1a18a86b1315f6945849940369
Reviewed-on: https://gerrit.chromium.org/gerrit/42296
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Commit-Queue: John Sheu <sheu@chromium.org>
Tested-by: John Sheu <sheu@chromium.org>