drm/i915: correct suspend/resume ordering

We need to save register state *after* idling GEM, clearing the ring,
and uninstalling the IRQ handler, or we might end up saving bogus
fence regs, for one.  Our restore ordering should already be correct,
since we do GEM, ring and IRQ init after restoring the last register
state, which prevents us from clobbering things.

I put this together to potentially address a bug, but I haven't heard
back if it fixes it yet.  However I think it stands on its own, so I'm
sending it in.

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