drm/i915: Refactor ctg+ trickle feed disable

Pull the code to disable trickle feed for all primary planes into a
separate function.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>