drm/i915: Fix 12bpc HDMI enable for IBX

Follow the procedure listed in Bspec to toggle the port enable bit off
and on when enabling HDMI with 12bpc and pixel repeat on IBX. The old
code didn't actually enable the port before "toggling" the bit back off,
so the whole workaround was essentially a nop.

Also take the opportunity to clarify the code by splitting the gmch
platforms to a separate (much more straightforward) function.

v2: Rebased due to crtc->config changes

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