backlight: fix adp8860_bl build errors

Add slab.h to fix part of build.
Add a parameter in backlight_device_register() call to fix part of build.

drivers/video/backlight/adp8860_bl.c:215: error: implicit declaration of function 'kzalloc'
drivers/video/backlight/adp8860_bl.c:215: warning: assignment makes pointer from integer without a cast
drivers/video/backlight/adp8860_bl.c:285: error: implicit declaration of function 'kfree'
drivers/video/backlight/adp8860_bl.c:673: warning: assignment makes pointer from integer without a cast
drivers/video/backlight/adp8860_bl.c:689: error: too few arguments to function 'backlight_device_register'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>