ARM: SAMSUNG: Fix compile error due to kfree

Fix following build error.
arch/arm/plat-samsung/dev-backlight.c: In function 'samsung_bl_set':
arch/arm/plat-samsung/dev-backlight.c:145: error: implicit declaration of function 'kfree'

Signed-off-by: Jonghwan Choi <jhbird.choi@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>