OMAPDSS: omap_dss_register_device() doesn't take display index

We used to have all the displays of the board in one list, and we made a
"displayX" directory in the sysfs, where X was the index of the display
in the list.

This doesn't work anymore with device tree, as there's no single list to
get the number from, and it doesn't work very well even with non-DT as
we need to do some tricks to get the index nowadays.

This patch changes omap_dss_register_device() so that it doesn't take
disp_num as a parameter anymore, but uses a private increasing counter
for the display number.

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