OMAPFB: search for default display with DT alias

Improve the search for the default display in two ways:
* compare the given display name to the display's alias
* if no display name is given, look for "display0" DT alias

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