drm: omapdrm: Move DISPC_CLK_SWITCH reg feature to struct dss_features

The register belongs to the DSS, move the feature to the dss_features
structure.

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