ARM: locomo: fix locomolcd_power declaration

The locomolcd driver has one remaining missing-prototype warning:

drivers/video/backlight/locomolcd.c:83:6: error: no previous prototype for 'locomolcd_power' [-Werror=missing-prototypes]

There is in fact an unused prototype with a similar name in a global
header, so move the actual one there and remove the old one.

Link: https://lore.kernel.org/r/20230927194844.680771-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>