backlight: s6e63m0: remove unnecessary NULL deference check

Remove unnecessary NULL deference check, because it was already checked in
s6e63m0_probe().  Also, POWER_IS_ON is replaced with
s6e63m0_power_is_on().

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>