OMAPDSS: panel-acx565akm: clean-up locking

The locking in the acx565akm panel driver was getting too complex. Clean
it up by making new functions, acx565akm_bl_get_intensity_locked and
acx565akm_bl_update_status_locked, which are called by the backlight
subsystem. This way the non-locked versions can be called by the panel's
other funcs, simplifying the lock management.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>