plugins: update mesa's vendor string in whitelist
plugins: update mesa's vendor string in whitelist

Mesa has updated its VA-API Gallium driver vendor string:

https://cgit.freedesktop.org/mesa/mesa/commit/?id=5db29d62ce1fefa3f2ee6e4a4688576fde4bde4a

This patch tries to cover both, the old and the new one.

https://bugzilla.gnome.org/show_bug.cgi?id=793386

ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi