OMAP: DSS2: Taal: add locks to taal_bl_update_status

taal_bl_update_status was missing locks to protect taal_data. This caused
a kernel crash randomly, as the code attempted to set the brightness while
the OMAP's DSI block was actually disabled.

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