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
work.
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