OMAPDSS: Add dss_get_default_display_name()

Add function dss_get_default_display_name() which returns the name of
the default display, given from the board file or via module parameters.
The default display name can be used by output drivers to decide which
display is the wanted one.

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