glx/dri: Filter out fbconfigs that don't have a supported pixmap format

For depth 30 in particular it's not uncommon for the DDX to not have
a configured pixmap format. Since the client expects to back both
GLXPixmaps and GLXPbuffers with X Pixmaps, trying to use an x2rgb10
fbconfig would fail along various paths to CreatePixmap. Filter these
fbconfigs out so the client can't ask for something that we know won't
4 jobs for dri-depth-30-fbconfig-fix in 2 minutes and 28 seconds (queued for 8 seconds)