drm/i915: set the right gen3 flip_done mode also at resume

Currently we've only frobbed this bit at irq_init time, but did
not restore it at resume time. Move it to the gen3 clock gating
function to fix this.

Notice while reading through code.

Cc: stable@vger.kernel.org (for 3.5 only)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>