Support sysfs backlight control for Sony laptops in xf86-video-intel

The sony_laptop kernel module (since v2.6.23) supports backlight control
via the sysfs interface:
/sys/class/backlight/sony

This patch will enable xf86-video-intel to use this backlight control method
for Sony VAIO Laptops with Intel integrated video.

src/i830_lvds.c

@@ -84,6 +84,7 @@ static char *backlight_interfaces[] = {
     "acpi_video1",
     "acpi_video0",
     "fujitsu-laptop",
+    "sony",
     NULL,
 };