drm: omapdrm: Use DRM core's atomic commit helper

The DRM core atomic helper now supports asynchronous commits natively.
The custom omapdrm implementation isn't needed anymore, remove it.

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