glx: Fix incorrect use of dri_interface.h version defines in driver probing.

glx: Fix incorrect use of dri_interface.h version defines in driver probing.

If we extend __DRI_CORE or __DRI_SWRAST in dri_interface.h to allow a
new version, it shouldn't make old server code retroactively require
the new version from swrast drivers.

Notably, new Mesa defines __DRI_SWRAST version 4, but we still want to
be able to probe version 1 drivers, since we don't use any features
beyond version 1 of the struct.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>