backlight: lcd: Move event handling into helpers

Move the handling of display updates to separate helper functions.
There is code for handling fbdev blank events and fbdev mode changes.
The code currently runs from fbdev event notifiers, which will be
replaced.

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/20250321095517.313713-8-tzimmermann@suse.de


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