media: vb2-dc: skip CPU sync in map/unmap dma_buf

This is rougly equivalent to ca0e68e21aae (drm/prime: skip CPU sync
in map/unmap dma_buf). The contig memory allocated is already device
coherent memory, so there is no point in doing a CPU sync when
mapping it to another device. Also most importers currently cache
the mapping so the CPU sync would only happen on the first import,
so we are better off with not pretending to do a cache synchronization
at all.

This gets rid of a lot of CPU overhead in uses where those dma-bufs
are regularily imported and detached again, like Weston is currently
doing in the DRM compositor.

Signed-off-by: Lucas Stach <[email protected]>
Acked-by: Marek Szyprowski <[email protected]>
Signed-off-by: Hans Verkuil <[email protected]>
[[email protected]: fix checkpatch warnings]
Signed-off-by: Mauro Carvalho Chehab <[email protected]>

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