drm/i915: Init infoframe vfuncs for DP encoders as well

DP ports may want to use the video DIP for SDP transmission, so let's
initialize the vfuncs for DP encoders as well. The only exception is
port A eDP prior to HSW as that one doesn't have a video DIP instance.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818134958.15502-6-ville.syrjala@linux.intel.com