glx: Delegate the core of glXGetScreenDriver to the GLX screen vtable

glx: Delegate the core of glXGetScreenDriver to the GLX screen vtable

This is a minor bugfix, in that the prior code required that the server
expose either XFree86-DRI or DRI2 to get the name; Xwayland exposed
neither, just DRI3. Now, for DRI2 and DRI3, we just ask the loader. It
also means we report "swrast" for the driver name when that's what we're
using, which is probably a good thing.

v2: Trust the driver name from the server for DRI2 (Michel Dänzer)

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7219>