OMAP: DSS2: remove sub-panel system

The system to allow panel drivers to exists as attached to ctrl drivers
did never work very well. It is not useed, and this patch removes it to
make the driver cleaner.

For now, controller drivers need to include also the panel driver code. In
the future a proper mechanism for this should be developed, perhaps by
creating busses for controllers.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>