drm/i915: refactor error detection & collection

This patch refactors the existing error detection and collection code,
placing most of it in i915_handle_error(). Additionally, we introduce a
work queue for scheduling post-crash tasks such as generating a uevent.
Using the uevent facility, userspace should be able to capture a
post-mortem dump for diagnostics.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ben Gamari <bgamari.foss@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>