drm/i915: Assert that the active request hasn't been signaled

As the request is not complete, it should not be signaled. Assert that
this is true before we process the request for a reset.

References: https://bugs.freedesktop.org/show_bug.cgi?id=99671


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170212172002.23072-1-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>