i915: suspend/resume interrupt state

In the KMS case, enter/leavevt won't fix up the interrupt handler for
us, so we need to do it at suspend/resume time.  Make sure we don't fail
the resume if the chip is hung either.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>