drm/i915: Nuke intel_ddi_get_encoder_port()

drm/i915: Nuke intel_ddi_get_encoder_port()

encoder->port works for FDI, and it also works for MST (regardless of
whether we're dealing with the "fake" MST encoder, or mst->primary).
So let's eliminate intel_ddi_get_encoder_port().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171027193128.14483-4-ville.syrjala@linux.intel.com