OMAPDSS: fix DPI and SDI device ids

The DPI and SDI platform devices are currently created with the ID of
-1. The ID doesn't currently affect anything.

However, we have added regulator supply entries for "omapdss_dpi.0" and
"omapdss_sdi.0" to the board files, although these supply entries are
not yet used. As the ID used for the devices is -1, these regulator
supply entries will not work.

To fix the issue, assign ID of 0 to the devices. In the future there may
be more than one DPI or SDI output, so it makes sense to have a proper
ID for them.

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