drm/i915: Move infoframe vfuncs into intel_digital_port

DP ports will also want to utilize the video DIP for SDP transmission.
So let's move the vfuncs into the dig_port.

v2: Rebase due to DDI changes

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/20170818134958.15502-5-ville.syrjala@linux.intel.com