misc: panel: Abstract temporary backlight handling

Currently the periodic scan timer is used for three purposes,
entangling keypad and display handling, which are both optional:
  1. Scanning the keypad,
  2. Flashing the backlight when a key is pressed,
  3. Disabling temporary backlighting after a fixed period of time.

Abstract the second purpose using a new lcd_poke() function.
Make the non-periodic temporary backlight handling independent from
keypad handling by converting it to a delayed workqueue.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>