drm/omap: DISPC: support double-pixel mode

We need double-pixel mode (pixel repetition) for interlace modes. This
patch adds the necessary support to DISPC to output double-pixel mode.

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