drm/i915: Untangle .fdi_link_train and cdclk vfunc setup

Split the .fdi_link_train and .modeset_commit_cdclk/.modeset_calc_cdclk
into two separate if ladders. Much easier to read when you're not
confusing two totally separate subjects.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-4-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>