drm/i915/guc: Fix revocation of non-persistent contexts

Patch which added graceful exit for non-persistent contexts missed the
fact it is not enough to set the exiting flag on a context and let the
backend handle it from there.

GuC backend cannot handle it because it runs independently in the
firmware and driver might not see the requests ever again. Patch also
missed the fact some usages of intel_context_is_banned in the GuC backend
needed replacing with newly introduced intel_context_is_schedulable.

Fix the first issue by calling into backend revoke when we know this is
the last chance to do it. Fix the second issue by replacing
intel_context_is_banned with intel_context_is_schedulable, which should
always be safe since latter is a superset of the former.

v2:
 * Just call ce->ops->revoke unconditionally. (Andrzej)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 45c64ecf

 ("drm/i915: Improve user experience and driver robustness under SIGINT or similar")
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: <stable@vger.kernel.org> # v6.0+
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Acked-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221003121630.694249-1-tvrtko.ursulin@linux.intel.com