OMAPDSS: add panel list

OMAPDSS: add panel list

We currently use the omapdss bus (which contains all the available
displays) to iterate the displays. As the omapdss bus is on its way out,
this needs to be changed.

Instead of using the dss bus to iterate displays, this patch adds our
own list of displays which we manage. The panels on the dss bus are
automatically added to this new list.

An "alias" field is also added to omap_dss_device. This field is
set to "display%d", the same way as omap_dss_device's dev name is set.
This alias is later used to keep backward compatibility, when the
embedded dev is no longer used.

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