drm/i915: Double check that the wait_request is not pending before warning

If we are busy, then we may have woken up the wait_request handler but
not yet serviced it before the hang check fires. So in hang check,
double check that the i915_gem_do_wait_request() is still pending the
wake-up before declaring all hope lost.

Fixes regression with e78d73b1.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30073


Reported-and-tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>