omap3_dss: define DSS_ONOFF

Add DSS_ONOFF to polarity defines

Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Acked-by: Anatolij Gustschin <agust@denx.de>