macintosh/via-pmu-backlight: 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>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220616170425.1346081-1-steve@sk2.org