OMAPDSS: DSS: remove legacy dss bus support

With all the old panels removed and all the old panel model APIs removed
from the DSS encoders, we can now remove the custom omapdss-bus which
was used in the old panel model.

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