gallium/drm: Refactor the stub screen create functions.

This gives me one place to document why it works this way.  This does make
the debug message a little less helpful ("etna" instead of "etnaviv" and
"vmwgfx" instead of "svga", but you should only be able to reach this when
doing something like trying the radeon/nouveau vdpau target on the wrong
DRM device for example.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6751>
1 file changed