Fix the skip disabled backlight device

Only raw backlight devices have the "enabled" file under device
directory.
So the commit

5c0d35c skip the disabled backlight device

affects all type of backlight device is wrong, it will drop out all
other type of backlight device except raw backlight interfaces.

To fix this, we just need to check the enabled file for raw backlight
interfaces.

Changes V2:

1. Declare isRawBacklightEnabled() as a static function
2. Using QFile constructor to pass the file path
3. Declare "QByteArray buffer" when it's really needed below
4. Remove "bool result", and just return true/false directly
5. Adding curly brackets to if statement

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>