drm/i915/fdi: move more FDI stuff to FDI link train hooks

Accept slight duplication in the fdi link train hooks in exchange for
simplification in ilk_pch_enable(). This lets us make
ivb_update_fdi_bc_bifurcation() static again, now in intel_fdi.c.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7984e670c6bb092d213d90f838d526d594d4a690.1629906431.git.jani.nikula@intel.com