OMAPDSS: add pdata->default_display_name

We can currently set the default display (i.e. the initial display) in
the omapdss platform data by using a pointer to the default
omap_dss_device. Internally omapdss uses the device's name to resolve
the default display.

As it's difficult to get the omap_dss_device pointer in the future,
after we've changed the omapdss device model, this patch adds a new way
to define the default display, by using the name of the display.

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