staging: fbtft: Stop using BL_CORE_DRIVER1

Leaking driver internal tracking into the already massively confusing
backlight power tracking is really confusing.

Luckily we have already a drvdata structure, so fixing this is really
easy.

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