Bug #7639: Only swap out pixmaps (rather than everything) on VT switch in EXA.

This is a new behavior for version 2.1 of EXA, and only takes effect if the
driver has requested that.  Otherwise, the previous behavior remains the same.

exa/exa_migration.c

@@ -249,7 +249,7 @@ exaCopyDirtyToFb (PixmapPtr pPixmap)
  * Called when the memory manager decides it's time to kick the pixmap out of
  * framebuffer entirely.
  */
-static void
+void
 exaPixmapSave (ScreenPtr pScreen, ExaOffscreenArea *area)
 {
     PixmapPtr pPixmap = area->privData;