dri: Fix __DRIconfig reporting of __DRI_ATTRIB_SWAP_METHOD

dri: Fix __DRIconfig reporting of __DRI_ATTRIB_SWAP_METHOD

The attribMap had two entries for this attribute, and
driGetConfigAttribIndex didn't return a proper value for this attribute.
Fix this, and also make sure we return SWAP_UNDEFINED for single-buffer
configs as required by the GLX_OML_swap_method spec.

Finally bump the dri core extension version to 2, indicating that we
correctly report __DRI_ATTRIB_SWAP_METHOD.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>