drm/i915: Abandon oom quickly if killed by a signal

Whilst waiting to obtain our locks for the last resort shrinking before
an oom, we check whether or not a fatal signal was pending. If there was,
we do not need to keep waiting as the oom will be aborted.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>