drm/i915: Assert breadcrumbs are correctly ordered in the signal handler

Inside the signal handler, we expect the requests to be ordered by their
breadcrumb such that no later request may be complete if we find an
earlier incomplete. Add an assert to check that the next breadcrumb
should not be logically before the current.

v2: Move the overhanging line into its own function and reuse it after
doing the insertion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190503152214.26517-1-chris@chris-wilson.co.uk