EXA: fix mixed-pixmaps issue where we could do accel with pixmap mapped

The problem occurs in the following situation:

1. Create Pixmap
2. Software fallback on pixmap: pExaPixmap->pDamage created
3. Accel to pixmap: driver pixmap created, exaPixmapIsOffscreen() now true
4. Software fallback on pixmap
4.1: PrepareAccess() from driver succeeds, so no DFS to system memory copy
4.2: Software rendering...
4.3: (pExaPixmap->pDamage && exaPixmapIsOffscreen()) is true, so we try
     to (incorrectly) UTS the data back to the driver pixmap, when we
     should just call the driver's FinishAccess() hook.
4.4: fail

The patch adds an additional check on pExaPixmap->offscreen, which should
(for the cases exaPixmapIsOffscreen() will return true) only be FALSE
when we haven't used PrepareAccess() on the pixmap.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Acked-By: Michel Dänzer <michel@daenzer.net>

[ Michel Dänzer: This is a 'backport' of commit
  647b79f87a from master - actually this patch
  was submitted first but didn't apply to master anymore, so technically the
  other change is a 'forward port' of this one ]

exa/exa_migration_mixed.c

@@ -202,7 +202,8 @@ void exaFinishAccess_mixed(PixmapPtr pPixmap, int index)
 {
     ExaPixmapPriv(pPixmap);
 
-    if (pExaPixmap->pDamage && exaPixmapIsOffscreen(pPixmap)) {
+    if (pExaPixmap->pDamage && exaPixmapIsOffscreen(pPixmap) &&
+	!pExaPixmap->offscreen) {
 	DamageRegionProcessPending(&pPixmap->drawable);
 	exaMoveInPixmap_mixed(pPixmap);
     }