[media] v4l: vb2: add support for shared buffer (dma_buf)

This patch adds support for DMABUF memory type in videobuf2. It calls relevant
APIs of dma_buf for v4l reqbuf / qbuf / dqbuf operations.
For this version, the support is for videobuf2 as a user of the shared buffer;
so the allocation of the buffer is done outside of V4L2. [A sample allocator of
dma-buf shared buffer is given at [1]]
[1]: Rob Clark's DRM:
   https://github.com/robclark/kernel-omap4/commits/drmplane-dmabuf


   [original work in the PoC for buffer sharing]

Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>