drm/vc4: Support the case where the DSI device is disabled

Having a device with a status property != "okay" in the DT is a valid
use case, and we should not prevent the registration of the DRM device
when the DSI device connected to the DSI controller is disabled.

Consider the ENODEV return code as a valid result and do not expose the
DSI encoder/connector when it happens.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180509130042.9435-5-boris.brezillon@bootlin.com