glamor: not working with EGL provider if glvnd is disabled.

Originally reported to me by email by Sylvain BERTRAND.

Latest patches by me and @ajax added a new GLX provider from Xwayland, which enabled by default when glamor is enabled.

UPD: it works for me, it appears he do not use libglvnd and use his variation of X server and mesa (with libglxvnd removed). I do not know, if this bug is valid.