drm/i915: Detect potential i915_reset_trylock() lockups

Use lockdep to warn before we wait indefinitely in case we may be
waiting indefinitely.
Suggested-by: Mika Kuoppala <mika.kuoppala@intel.com>
References: 2caffbf1 ("drm/i915: Revoke mmaps and prevent access to fence registers across reset")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190212130831.14425-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_reset.c

@@ -1305,6 +1305,9 @@ int i915_reset_trylock(struct drm_i915_private *i915)
 	struct i915_gpu_error *error = &i915->gpu_error;
 	int srcu;
 
+	might_lock(&error->reset_backoff_srcu);
+	might_sleep();
+
 	rcu_read_lock();
 	while (test_bit(I915_RESET_BACKOFF, &error->flags)) {
 		rcu_read_unlock();