drm/etnaviv: Make the "core" clock mandatory

It is always present. It was documented as mandatory prior to
commit 90aeca87 ("dt-bindings: display: Convert etnaviv to
json-schema").
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_gpu.c

@@ -1796,7 +1796,7 @@ static int etnaviv_gpu_platform_probe(struct platform_device *pdev)
 	if (IS_ERR(gpu->clk_bus))
 		return PTR_ERR(gpu->clk_bus);
 
-	gpu->clk_core = devm_clk_get_optional(&pdev->dev, "core");
+	gpu->clk_core = devm_clk_get(&pdev->dev, "core");
 	DBG("clk_core: %p", gpu->clk_core);
 	if (IS_ERR(gpu->clk_core))
 		return PTR_ERR(gpu->clk_core);