OMAP: DSS2: Taal: Change DSI bus locking to avoid deadlock in ESD work

Move dsi_bus_lock/unlock() out of taal_power_on/off() to avoid deadlock
in taal_esd_work().

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>