drm/i915/fdi: move intel_fdi_link_freq() to intel_fdi.[ch]

There's no performance reason to have it as static inline; move it out
of intel_display_types.h to reduce clutter and dependency on i915_drv.h.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8c9bb23d92878deb1ecc75427ec6648bd3505816.1629281426.git.jani.nikula@intel.com