drm/i915: Return error in i915_gem_set_to_gtt_domain if we're not in the GTT.

It's only for flushing caches appropriately for GTT access, not for actually
getting it there.  Prevents potential smashing of cpu read/write domains on
unbound objects.

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