[patch v2] i915: take struct_mutex lock in intel_setup_overlay()

[Dan Carpenter <error27@gmail.com>]

I changed the drm_gem_object_unreference() to
drm_gem_object_unreference_unlocked().

"reg_bo" is a local variable so there are no synchronization issues, but
the problem is that if struct_mutex isn't held, it triggers a BUG_ON()
in drm_gem_object_free().

I also took the "struct_mutex" when calling i915_gem_object_pin().
The i915_gem_object_pin() function calls i915_gem_object_bind_to_gtt()
which unreferences stuff if we are low on gtt space.  The truth is that 
if we run out of gtt space on this code path, we're sunk already 
regardless of the locking.  But we should take the lock anyway for
consistency and to make the static checkers happy.

That call tree is:
i915_gem_object_pin()
    => i915_gem_object_bind_to_gtt()
        => i915_gem_evict_something()
            => i915_gem_retire_requests()
                => i915_gem_retire_request()
                    => drm_gem_object_unreference()

intel_setup_overlay() is always called with "struct_mutex" unlocked.
There is only one call tree and it is:

i915_load_modeset_init()
    => intel_modeset_init()
           => intel_setup_overlay()

Signed-off-by: Dan Carpenter <error27@gmail.com>
---
V2:  The code is the same, but the change log is updated.
No one has complained about hitting the BUG_ON() and I can't tested it
myself, so this patch is probably linux-next material.

diff --git a/drivers/gpu/drm/i915/intel_overlay.c b/drivers/gpu/drm/i915/intel_overlay.c
index 6d524a1..a103582 100644
--- a/drivers/gpu/drm/i915/intel_overlay.c
+++ b/drivers/gpu/drm/i915/intel_overlay.c
@@ -1347,7 +1347,9 @@ void intel_setup_overlay(struct drm_device *dev)
 	overlay->reg_bo = to_intel_bo(reg_bo);
 
 	if (OVERLAY_NONPHYSICAL(dev)) {
+		mutex_lock(&dev->struct_mutex);
 		ret = i915_gem_object_pin(reg_bo, PAGE_SIZE);
+		mutex_unlock(&dev->struct_mutex);
 		if (ret) {
                         DRM_ERROR("failed to pin overlay register bo\n");
                         goto out_free_bo;
@@ -1385,7 +1387,7 @@ void intel_setup_overlay(struct drm_device *dev)
 	return;
 
 out_free_bo:
-	drm_gem_object_unreference(reg_bo);
+	drm_gem_object_unreference_unlocked(reg_bo);
 out_free:
 	kfree(overlay);
 	return;