drm/i915: Inline the required bits of intel_ddi_post_disable() into intel_ddi_fdi_post_disable()

To untangle the mess that is intel_ddi_post_disable() move the the bits
needed by FDI into intel_ddi_fdi_post_disable(). This way we can stop
worrying about FDI in intel_ddi_post_disable().

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010121207.570-5-ville.syrjala@linux.intel.com