drm/etnaviv: remove the need for a gpu-subsystem DT node

The module autoloading can be triggered through the GPU core nodes
and the necessary platform device for the DRM toplevel device will
be instantiated on module init.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>