drm/i915: make FDI training a display function

Rather than branching in ironlake_pch_enable, add a new train_fdi
function to the display function pointer struct and use it instead.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>