drm/i915: Handle async cancellation in sentinel assert

With the watchdog cancelling requests asynchronously to preempt-to-busy we
need to relax one assert making it apply only to requests not in error.

v2:
 * Check against the correct request!

v3:
 * Simplify the check to avoid the question of when to sample the fence
   error vs sentinel bit.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210324121335.2307063-5-tvrtko.ursulin@linux.intel.com