drm/i915: Wait for the GPU whilst shrinking, if truly desperate.

By idling the GPU and discarding everything we can when under extreme
memory pressure, the number of OOM-killer events is dramatically
reduced. For instance, this makes it possible to run
firefox-planet-gnome.trace again on my swapless 512MiB i915.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>