drm/i915: split display functions by chip type

This patch splits out several of the display functions into a separate
display function table to avoid tons of chipset specific if..else
if..else if blocks all over.  There are more opportunities for this
(some noted in the structure defintition); so more cleanup patches will
follow.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>