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

traps.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/20240731130720.1148872-2-tzimmermann@suse.de