macintosh/via-pmu-backlight: Use backlight power constants

Replace FB_BLANK_ constants with their counterparts from the
backlight subsystem. The values are identical, so there's no
change in functionality or semantics.

via-pmu-backlight.c already includes backlight.h where the
BACKLIGHT constants are defined.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240731130720.1148872-3-tzimmermann@suse.de