drm/etnaviv: Check for platform_device_register_simple() failure

platform_device_register_simple() may fail, so we should better
check its return value and propagate it in the case of error.

Cc: <stable@vger.kernel.org>
Fixes: 246774d1 ("drm/etnaviv: remove the need for a gpu-subsystem DT node")
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>