drm/i915: add GEM GTT mapping support

Use the new core GEM object mapping code to allow GTT mapping of GEM
objects on i915.  The fault handler will make sure a fence register is
allocated too, if the object in question is tiled.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>