backlight: Move blank-state handling into helper

Move the handling of blank-state updates into a separate helper,
so that is can be called without the fbdev event. No functional
changes.

As a minor improvement over the original code, the update replaces
manual locking with a guard.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: "Daniel Thompson (RISCstar)" <danielt@kernel.org>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20250321095517.313713-6-tzimmermann@suse.de


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