drm: omapdrm: Move reg_fields to dispc_features structure

The reg_fields feature describes DISPC registers only. Move it from the
omap_dss_features structure to the dispc_features structure.

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