egl/x11: don't populate dri2_dpy->dri2_loader_extension

egl/x11: don't populate dri2_dpy->dri2_loader_extension

Analogous to the earlier android and wayland patches. As we're here we
can drop exposing the old version of the extension.

Any dri loader/driver interface use lower bound checking thus exposing
dri2 loader v3 to a v2 capable driver is perfectly normal.

v2: Preserve compat with dri2_minor < 1. The driver does not know if
there is a protocol to manage getBuffersWithFormat(). It's up-to the
loader to expose the vfunc if there is one. (Kristian)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>