OMAPDSS: add output->dispc_channel

The DISPC channel used for each output is currently passed in panel
platform data from the board files.

To simplify this, and to make the panel drivers less dependent on OMAP,
this patch changes omapdss to resolve the channel independently. The
channel is resolved based on the OMAP version and, in case of DSI, the
DSI module id. This resolved channel is stored into a new field in
output, dispc_channel.

The few places where dssdev->channel was used are changed to use
output->recommended_channel. After this patch, dssdev->channel is
obsolete.

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