fbdev: omap2/omapfb: Replace use of fb_blank with backlight helpers

Replace the use of struct backlight_properties.fb_blank with backlight
helpers. This effects testing if the backlight is blanked and reading
the backlight's brightness level.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20240319093915.31778-5-tzimmermann@suse.de


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