drm/i915: Assert mutex_is_locked on context lookup

Because our context refcounting doesn't grab a ref at lookup time, it is
unsafe to do so without the lock.

NOTE: We don't have an easy way to put the assertion in the lookup
function which is where this really belongs. Context switching is good
enough because it actually asserts even more correctness by protecting
the default_context.
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
[danvet: s/BUG/WARN/]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -463,6 +463,8 @@ int i915_switch_context(struct intel_ring_buffer *ring,
 	if (dev_priv->hw_contexts_disabled)
 		return 0;
 
+	WARN_ON(!mutex_is_locked(&dev_priv->dev->struct_mutex));
+
 	if (ring != &dev_priv->ring[RCS])
 		return 0;