drm/i915: fix flags in dma buf exporting

As pointed out by Seung-Woo Kim this should have been
passing flags like nouveau/radeon have.

Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>