OMAPDSS: change default_device handling

We currently have a two ways to set a "default panel device" for dss, to
which the overlays are connected when the omapdss driver is loaded:

- in textual format (name of the display) as cmdline parameter
- as a pointer to the panel device from board file via pdata

The current code handles this in a bit too complex way by using both of
the above methods during runtime. However, with DT we don't have pdata
anymore, so the code handling the second case won't work anymore. The
current code has also the problem that it modifies the platform_data.

This patch simplifies the code a bit by using the pointer method only
inside the probe function, and stores the name of the panel device. This
way we only need to handle the textual format during operation and also
avoid modifying the platform_data.

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