drm: Rename drm_connector_unplug_all() to drm_connector_unregister_all()

Current name is a bit misleading because what that helper function
really does it calls drm_connector_unregister() for all connectors.

This all has nothing to do with hotplugging so let's name things
properly.

And while at it remove potentially dangerous locking around
drm_connector_unregister() in rcar_du_remove() as mentioned
in kerneldoc for drm_connector_unregister_all().

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: linux-renesas-soc@vger.kernel.org
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1458722577-20283-2-git-send-email-abrodkin@synopsys.com