backlight: backlight: Drop extern from prototypes

No need to put "extern" in front of prototypes. While touching the
prototypes adjust indent to follow the kernel style.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>