regulator: rpi-panel-attiny: Use backlight helper

regulator: rpi-panel-attiny: Use backlight helper

backlight_properties.fb_blank is deprecated. The states it represents
are handled by other properties; but instead of accessing those
properties directly, drivers should use the helpers provided by
backlight.h.

Instead of retrieving the backlight brightness in struct
backlight_properties manually, and then checking whether the backlight
should be on at all, use backlight_get_brightness() which does all
this and insulates this from future changes.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220607185304.1128962-1-steve@sk2.org
Signed-off-by: Mark Brown <broonie@kernel.org>