OMAPDSS: export dss_get_def_display_name()

Export dss_get_def_display_name() with the name of
omapdss_get_def_display_name() so that omapfb can use it after the next
patch which moves default display handling to omapfb.

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