compat: add helpers to aid backport of generic DMA changes for v4l

compat: add helpers to aid backport of generic DMA changes for v4l

Commit c60520fa needs to be reverted for older kernels because
although we can backport some large changes to DMA that
ended up extending some core dma data structures, for
details see bca0fa5f as an example. To *aid* with the driver
changes required add a helper that lets us call what we used
to do, thus requiring a smaller delta on the upstream code.

We're only adding this for >= 3.2 as this is required only
for media which are also only supporting on >= 3.2.

commit c60520fa50cd86d64bc8ebb34300ddc4ca91393d
Author: Marek Szyprowski <m.szyprowski@samsung.com>
Date:   Thu Jun 14 11:32:21 2012 -0300

    [media] v4l: vb2-dma-contig: let mmap method to use dma_mmap_coherent call

    Let mmap method to use dma_mmap_coherent call.  Moreover, this patch removes
    vb2_mmap_pfn_range from videobuf2 helpers as it was suggested by Laurent
    Pinchart.  The function is no longer used in vb2 code.

    Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
    Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
    Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
    Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
    Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>