drm: omapdrm: Move all debugfs code from core to dss

debugfs code is spread between the core and dss drivers. In preparation
for removal of the core driver, move it all to the dss driver.

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