drm/i915: Match GTT space sanity checker with implementation

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: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Also adjust the comment per irc discussion with Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>