drm: omapdrm: dss: Store the debugfs root directory in struct dss_device

As part of an effort to remove the usage of global variables in the
driver, store the debugfs root directory in the dss_device structure
instead of a global variable.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>