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>

include/GL/internal/dri_interface.h

@@ -726,9 +726,12 @@ struct __DRIuseInvalidateExtensionRec {
 
 /**
  * This extension defines the core DRI functionality.
+ *
+ * Version >= 2 indicates that getConfigAttrib with __DRI_ATTRIB_SWAP_METHOD
+ * returns a reliable value.
  */
 #define __DRI_CORE "DRI_Core"
-#define __DRI_CORE_VERSION 1
+#define __DRI_CORE_VERSION 2
 
 struct __DRIcoreExtensionRec {
     __DRIextension base;