OMAPDSS: SDI: remove code related to old panel model

Now that the old panel drivers have been removed, we can remove the
old-model API and related code from the DSS encoder drivers.

This patch removes the code from the SDI driver.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>