[ARM] pxafb: avoid the racing condition in pxafb_smart_thread

fbi->state change shall really be protected by fbi->ctrlr_lock, where
the change is sheltered. There is a possibility that pxafb_smart_thread
will start update the LCD panel when fbi->state == C_ENABLE, while
all other initialization isn't done.

Signed-off-by: Eric Miao <eric.miao@marvell.com>