drm/i915: Move intel_ddi_get_crtc_new_encoder() out from ddi code

The ddi code no longer uses intel_ddi_get_crtc_new_encoder(). Move it
elsewhere where we have some users left.

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/20180518152931.13104-1-ville.syrjala@linux.intel.com