xwayland: Enable DRI3 for glamor

glamor_fds_from_pixmap() will bail out early if DRI3 is not enabled,
unfortunately Xwayland's glamor code would not set it as enabled which
would lead to blank pixmaps when using texture from pixmap.

Make sure to mark DRI3 as enabled from glamor_egl_screen_init() in
Xwayland.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107287


Fixes: c8c276c9 ("glamor: Implement PixmapFromBuffers and BuffersFromPixmap")
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>