drm/armada: de-midlayer armada

Now that the drm_connector_register() is gone from tda998x, we can
remove the mid-layer from armada-drm, eliminating the load, unload,
debugfs_init, and debugfs_cleanup callbacks from armada's drm_driver
structure.  No functional changes.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>