OMAPDSS: add 'label' support for DT

Add support to get the label (i.e. a "nickname") for a display from the
DT data. If there is no label defined, use the display's alias (e.g.
'display0') as a name.

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