omapdrm: panel: fix compatible vendor string for td028ttec1

The vendor name was "toppoly" but other panels and the vendor list
have defined it as "tpo". So let's fix it in driver and bindings.

We keep the old definition in parallel to stay compatible with
potential older DTB setup.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>