drm/i915: Make a single set-to-gtt-domain path.

This fixes failure to flush caches in the relocation update path, and
failure to wait in the set_domain ioctl, each of which could lead to incorrect
rendering.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>