glx: Work around a GLX_OML swap method in older dri drivers

The swapMethod config member would typically contain an arbitrary value
on older dri drivers. Fix this so that if we detect an illegal value,
return GLX_SWAP_UNDEFINED_OML.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 0fc26310)