drm/i915: Split intel_ddi_post_disable() into DP vs. HDMI variants

To clean up the mess in intel_ddi_post_disable() split it into two
clean variants for HDMI and DP.

v2: Rebase due to MST DPMS changes

Reviewed-by: Jani Nikula <jani.nikula@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010121207.570-6-ville.syrjala@linux.intel.com