drm/i915: reject GTT domain in relocations

This confuses our domain tracking and can (for gtt write domains) lead
to a subsequent oops.

Tested by tests/gem_exec_bad_domains from i-g-t.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>