drivers: update link to panfrost

The Panfrost docs are now upstream in the Mesa docs, so we can link
directly there instead of via a redirect.

data/drivers.yml

@@ -77,7 +77,7 @@
       reverse-engineered, community-developed driver, and is not
       endorsed by NVIDIA.
   - name: Panfrost
-    external_url: https://panfrost.freedesktop.org/
+    docs_article: drivers/panfrost.html
     description: >-
       Panfrost is a free and open source driver for the ARM Mali Midgard
       and Bifrost GPUs. It's a reverse-engineered, community-developed