lwmon5 SYSMON POST: fix backlight control

If the LWMON5 config has SYSMON POST among CONFIG_POSTs which may be
run on the board, then the SYSMON POST controls the display backlight
(doesn't switch backlight ON if POST FAILED, and does switch the
backlight ON if PASSED).

If not, then the video driver controls the display backlight (just
switch ON the backlight upon initialization).

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>