drm/i915: Mark imported dma-buf objects as being coherent

A foreign dma-buf does not share our cache domain tracking, and we rely
on the producer ensuring cache coherency. Marking them as being in the
CPU domain is incorrect.

v2: Add commentary about the GTT domain. This is not the best place for
it, but pending an actual overhaul of our domain tracking and explaining
each one, this comment should help the next reader...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469002875-2335-7-git-send-email-chris@chris-wilson.co.uk