drivers/video/backlight/ams369fg06.c: make power_on() call optional

This patch makes power_on() call optional.  The voltage source can be
provided to some boards using ams369fg06 panel, thus in this case, power
on/off sequence is not necessary.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>