drm/i915/gt: Report context-is-closed prior to pinning

Our assertion caught that we do try to pin a closed context if userspace
is viciously racing context-closure with execbuf, so make it fail
gracefully.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1492
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200320130159.3922-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_context.c

@@ -97,8 +97,6 @@ int __intel_context_do_pin(struct intel_context *ce)
 {
 	int err;
 
-	GEM_BUG_ON(intel_context_is_closed(ce));
-
 	if (unlikely(!test_bit(CONTEXT_ALLOC_BIT, &ce->flags))) {
 		err = intel_context_alloc_state(ce);
 		if (err)
@@ -114,6 +112,11 @@ int __intel_context_do_pin(struct intel_context *ce)
 		goto out_release;
 	}
 
+	if (unlikely(intel_context_is_closed(ce))) {
+		err = -ENOENT;
+		goto out_release;
+	}
+
 	if (likely(!atomic_add_unless(&ce->pin_count, 1, 0))) {
 		err = intel_context_active_acquire(ce);
 		if (unlikely(err))