backlight: fix blanking for L4F00242T03 LCD

The LCD was turned on if the variable power was > 0, but that was
incorrect.  The LCD has to be turned on in NORMAL and UNBLANK case.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>