Switch the default migration heuristic for EXA to "always".

This has been what has been used the most successfully post-damagetrack.
The current thinking is that:
1) We should be able to accelerate basically everything.  So we don't need to
   try to migrate trees of pixmaps permanently out of framebuffer to speed
   CPU drawing up.
2) Migration is cheaper in the thrashing case, so we don't want to go to a lot
   of effort to try (and fail badly) to find a working set.

exa/exa.c

@@ -622,7 +622,7 @@ exaDriverInit (ScreenPtr		pScreen,
 
     pScreen->devPrivates[exaScreenPrivateIndex].ptr = (pointer) pExaScr;
 
-    pExaScr->migration = ExaMigrationSmart;
+    pExaScr->migration = ExaMigrationAlways;
 
     exaDDXDriverInit(pScreen);