drm/i915: split fdi code out from intel_display.c

This just refactors out the fdi code to a separate file.

Signed-off-by: Dave Airlie <airlied@redhat.com>
[Jani: cleaned up intel_fdi.h a bit.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f9d52c3d91f0973af308ede16e266fc9b753ecf9.1610622609.git.jani.nikula@intel.com