auxdisplay: ht16k33: Replace use of fb_blank with backlight helper

Replace the use of struct backlight_properties.fb_blank with a call
to backlight_get_brightness(). The helper implements similar logic
as the driver's function. It also accounts for BL_CORE_SUSPENDED for
drivers that set BL_CORE_SUSPENDRESUME. Ht16k33 doesn't use this, so
there's no change in behaviour here.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Robin van der Gracht <robin@protonic.nl>
Cc: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Robin van der Gracht <robin@protonic.nl>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20240319093915.31778-2-tzimmermann@suse.de


Signed-off-by: Lee Jones <lee@kernel.org>