MAINTAINERS: Add dri-devel for backlight subsystem patches

For the same reasons we've added dri-devel for all fbdev patches: Most
of the actively developed drivers using this infrastructure are in
drivers/gpu/. It just makes sense to cross-post patches and keep
aligned. And total activity in the backlight subsystem is miniscule
compared to drm overall.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>