Grafana dashboard: fix GPU Power Total (NVIDIA#146)

Make this an "instant" query, that is, calculate the sum over the latest
values only (the values listed under "current" in the GPU Power Usage
panel to the left)

Previously, it would sum all the values in the range, resulting in an
absurdly high value (which could perhaps be scaled to kWh, but is
definitly not W any more)

See
https://grafana.com/grafana/dashboards/12239-nvidia-dcgm-exporter-dashboard/?tab=reviews
for at least three comments hinting at this problem.

grafana/dcgm-exporter-dashboard.json

@@ -354,8 +354,10 @@
       "targets": [
         {
           "expr": "sum(DCGM_FI_DEV_POWER_USAGE{instance=~\"${instance}\", gpu=~\"${gpu}\"})",
+          "instant": true,
           "interval": "",
           "legendFormat": "",
+          "range": false,
           "refId": "A"
         }
       ],