drm/etnaviv: Warn when probing on NPUs

Userspace is still not making full use of the hardware, so we don't know
yet if changes to the UAPI won't be needed. Warn about it.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>