backlight: Remove obsolete comment for ->state

Jani spotted this when reviewing my earlier patch to remove the driver
internal usage of this field in:

  Commit 3cf91ada ("backlight: Nuke BL_CORE_DRIVER1")

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>