drm/i915: Restrict sentinel requests further

Disallow sentinel requests follow previous sentinels to make request
cancellation work better when faced with a chain of requests which have
all been marked as in error.

Because in cases where we end up with a stream of cancelled requests we
want to turn off request coalescing so they each will get individually
skipped by the execlists_schedule_in (which is called per ELSP port, not
per request).

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
[danvet: Fix typo in the commit message that Matthew spotted.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210324121335.2307063-4-tvrtko.ursulin@linux.intel.com