drm: omapdrm: dss: Set the DMA coherent mask

When merging the omapdrm and omapdss drivers the omapdrm virtual
platform device will disappear, and the omapdss platform device will be
used for DMA memory allocation. To prepare for that, set the DMA
coherent mask for the device.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>