drm: omapdrm: Remove dss_features.h

The header file only contains four macros, two of which are never used.
Move the other two to dss.h and remove dss_features.h.

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