drm/omap: set DRIVER_ATOMIC for omapdrm

omapdrm supports atomic modesetting, and it seems to work ok. So let's
set the flag to enable the atomic modesetting API support.

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