OMAP: DSS2: Fix def_disp module param description

MODULE_PARM_DESC() takes the name of the actual module parameter, not the
name of the variable, as input. Fix the module parameter description for
def_disp.

Signed-off-by: Jani Nikula <jani@nikula.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>