ARM: OMAP2+: Don't register omapdss device for omapdrm

The omapdrm driver doesn't need the omapdss device anymore. Although it
can't be removed completely as the fbdev driver still requires it, we
can condition its registration to the usage of the omapfb driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>