OMAPDSS: Taal: remove multi-panel support

Taal panel driver was originally meant to support multiple different DSI
command mode panel models. This never realized, and the multi-panel
support code is lying there unused, making the driver more difficult to
maintain.

This patch removes the multi-panel support from Taal driver.

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