drm/i915: Remove duplicate DDI enabling logic from MST path

The logic to enable a DDI in intel_mst_pre_enable_dp() is essentially
the same as in intel_ddi_pre_enable_dp(). So reuse the latter function
by calling the post_disable hook on the intel_dig_port instead of
duplicating that code.

v2: Don't oops because of a NULL encoder->crtc. (Ville)
v3: Warn for MST + PORT_E too. (Ville)
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302125857.14665-8-ander.conselvan.de.oliveira@intel.com