drm/i915: conditionally disable pch resources in ilk_crtc_disable

Simlar to how disable already works on haswell. This is possible
since we now carefully track the pch state in the pipe config.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>