drm/omap: dsi: drop unused get_te()

The get_te() callback is not used, so we can drop the
custom API.

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/20201215104657.802264-19-tomi.valkeinen@ti.com