drm/i915: don't enable the plane too early in i9xx_crtc_mode_set

This is horrible lore and we should be able to get rid of it now
that the lvds/pfit handling code actually does the right thing.

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