backlight: fix oops in __mutex_lock_slowpath during head /sys/class/graphics/fb0/*

Seems like not all drivers use the framebuffer_alloc() function and won't
have an initialized mutex.  But those don't have a backlight, anyway.
Signed-off-by: Michael Hanselmann <linux-kernel@hansmi.ch>
Cc: Olaf Hering <olaf@aepfle.de>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Cc: Daniel R Thompson <daniel.thompson@st.com>
Cc: Jon Smirl <jonsmirl@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/video/fbsysfs.c

@@ -397,6 +397,12 @@ static ssize_t store_bl_curve(struct class_device *class_device,
 	u8 tmp_curve[FB_BACKLIGHT_LEVELS];
 	unsigned int i;
 
+	/* Some drivers don't use framebuffer_alloc(), but those also
+	 * don't have backlights.
+	 */
+	if (!fb_info || !fb_info->bl_dev)
+		return -ENODEV;
+
 	if (count != (FB_BACKLIGHT_LEVELS / 8 * 24))
 		return -EINVAL;
 
@@ -430,6 +436,12 @@ static ssize_t show_bl_curve(struct class_device *class_device, char *buf)
 	ssize_t len = 0;
 	unsigned int i;
 
+	/* Some drivers don't use framebuffer_alloc(), but those also
+	 * don't have backlights.
+	 */
+	if (!fb_info || !fb_info->bl_dev)
+		return -ENODEV;
+
 	mutex_lock(&fb_info->bl_mutex);
 	for (i = 0; i < FB_BACKLIGHT_LEVELS; i += 8)
 		len += snprintf(&buf[len], PAGE_SIZE,