drm/i915: dev_priv cleanup in i915_gem_gtt.c

Started with removing INTEL_INFO(dev) and cascaded into a quite
big trickle of function prototype changes. Still, I think it is
for the better.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>