gma500: fix compile warnings when CONFIG_BACKLIGHT_CLASS_DEVICE is not defined

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
[Fixed up for other changes, and tidied some existing variable names]
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>