drm/i915: split irq handling into per-chipset functions

Set the IRQ handling functions in driver load so they'll just be used
directly, rather than branching over most of the code in the chipset
functions.

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