drm/i915/fdi: hide struct intel_fdi_funcs

The struct is only needed in intel_fdi.c, move it there.

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/a6c524a9abb74be9b4d6a084de5461404b823a10.1643896905.git.jani.nikula@intel.com