drm/i915: extract ironlake_fdi_pll_disable

Simply to make the ilk+ crtc disable path clearer and more symmetric
with the enable function.

Also switch to intel_crtc for the enable function.

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