If we believe that the device can cross cache domains in its prefetcher
(i.e. we allow neighbouring pages in different domains), we don't supply
a color_adjust callback. Use the presence of this callback to better
determine when we should be verifying that the GTT space we just
used is valid.

v2: Remove the superfluous struct drm_device function param as well.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
[danvet: Also adjust the comment per irc discussion with Chris.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>