drm/i915: Use ORIGIN_CPU for fb invalidation from pwrite

drm/i915: Use ORIGIN_CPU for fb invalidation from pwrite

As pwrite does not use the fence for its GTT access, and may even go
through a secondary interface avoiding the main VMA, we cannot treat the
write as automatically invalidated by the hardware and so we require
ORIGIN_CPU frontbufer invalidate/flushes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-4-chris@chris-wilson.co.uk
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>