drm/i915: move a gtt flush to the correct place

No functional change, because gtt flushing is a no-op. Still, try
to keep the bookkeeping accurate. The if is still slightly wrong
for with execbuf2 even i915-class hw doesn't always need a fence
reg for gpu access. But that's for somewhen lateron.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>