drm/i915: Implement an oom-notifier for last resort shrinking

Before the process killer is invoked, oom-notifiers are executed for one
last try at recovering pages. We can hook into this callback to be sure
that everything that can be is purged from our page lists, and to give a
summary of how much memory is still pinned by the GPU in the case of an
oom. This should be really valuable for debugging OOM issues.

Note that the last-ditch effort call to shrink_all we've previously
called from our normal shrinker when we could free as much as the vm
demaned is moved into the oom notifier. Since the shrinker accounting
races against bind/unbind operations we might have called shrink_all
prematurely, which this approach with an oom notifier avoids.

References: https://bugs.freedesktop.org/show_bug.cgi?id=72742


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: lu hua <huax.lu@intel.com>
[danvet: Bikeshed logical | into || and pimp commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>