drm/omap: dsi: drop useless sync()

The DSI sync() function only locks the bus and then releases
it again. Currently the only invocation is directly before
update(), which locks the bus anyways.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-21-tomi.valkeinen@ti.com