drm/etnaviv: split out wait for gpu idle

Split out into a new externally visible function, as the IOMMUv2
code needs this functionality, too.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_gpu.h

@@ -214,6 +214,7 @@ struct etnaviv_cmdbuf *etnaviv_gpu_cmdbuf_new(struct etnaviv_gpu *gpu,
 void etnaviv_gpu_cmdbuf_free(struct etnaviv_cmdbuf *cmdbuf);
 int etnaviv_gpu_pm_get_sync(struct etnaviv_gpu *gpu);
 void etnaviv_gpu_pm_put(struct etnaviv_gpu *gpu);
+int etnaviv_gpu_wait_idle(struct etnaviv_gpu *gpu, unsigned int timeout_ms);
 
 extern struct platform_driver etnaviv_gpu_driver;