drm/i915: grab dev->struct_mutex around framebuffer_init

We look at gem state (like obj->tiling/obj->stride), we better have
the relevant locks.

Right now this doesn't matter much since most of these checks are
a curtesy to safe buggy userspace, but I'd like to freeze the tiling
once we have framebuffer objects attached. And then locking matters.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -7285,14 +7285,21 @@ intel_framebuffer_create(struct drm_device *dev,
 		return ERR_PTR(-ENOMEM);
 	}
 
+	ret = i915_mutex_lock_interruptible(dev);
+	if (ret)
+		goto err;
+
 	ret = intel_framebuffer_init(dev, intel_fb, mode_cmd, obj);
-	if (ret) {
-		drm_gem_object_unreference_unlocked(&obj->base);
-		kfree(intel_fb);
-		return ERR_PTR(ret);
-	}
+	mutex_unlock(&dev->struct_mutex);
+	if (ret)
+		goto err;
 
 	return &intel_fb->base;
+err:
+	drm_gem_object_unreference_unlocked(&obj->base);
+	kfree(intel_fb);
+
+	return ERR_PTR(ret);
 }
 
 static u32
@@ -9989,6 +9996,8 @@ int intel_framebuffer_init(struct drm_device *dev,
 	int pitch_limit;
 	int ret;
 
+	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
+
 	if (obj->tiling_mode == I915_TILING_Y) {
 		DRM_DEBUG("hardware does not support tiling Y\n");
 		return -EINVAL;