drm/i915: don't use gtt_pwrite on LLC cached objects

~120 µs instead fo ~210 µs to write 1mb on my snb. I like this.

Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>