drm/msm/adreno: Check for zap node availability

This should allow disabling the zap node via an overlay, for slbounce.
Suggested-by: Nikita Travkin <nikita@trvn.ru>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/594572/

drivers/gpu/drm/msm/adreno/adreno_gpu.c

@@ -46,7 +46,7 @@ static int zap_shader_load_mdt(struct msm_gpu *gpu, const char *fwname,
 	}
 
 	np = of_get_child_by_name(dev->of_node, "zap-shader");
-	if (!np) {
+	if (!of_device_is_available(np)) {
 		zap_available = false;
 		return -ENODEV;
 	}