locking/drm: Fix i915_gem_shrinker_lock() locking

Mike Krinkin reported hangs in the DRM code and bisected it to:

3ab7c086d5ec72585ef0 ("locking/drm: Kill mutex trickery")

Hugh Dickins observed:

"i915_gem_shrinker_lock() is broken: but copy the pattern from
msm_gem_shrinker_lock() and it's okay - patch below."

Pick up the fix in isolation to make sure the bug is fixed, cleanup
patch will follow up.

Originally-From: Hugh Dickins <[e-mail]>
Reported-by: Hugh Dickins <[e-mail]>
Reported-by: Mike Krinkin <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Paul E. McKenney <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

drivers/gpu/drm/i915/i915_gem_shrinker.c

···
 static bool i915_gem_shrinker_lock(struct drm_device *dev, bool *unlock)
 {
 	if (!mutex_trylock(&dev->struct_mutex))
-		*unlock = false;
+		return false;
 
+	*unlock = true;
 	return true;
 }