drm: omapdrm: dss: Split operations out of dss_features structure

Move the two function pointers to a new dss_ops structure. This will
allow merging the dss_features and omap_dss_features structures without
having to expose the DPI source selection and LCD clock muxing functions
in header files.

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