drm/i915: split page flip queueing into per-chipset functions

This makes things a little clearer and prevents us from running old code
on a new chipset that may not be supported.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewied-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Keith Packard <keithp@keithp.com>