dix: allow pixmap dirty helper to be used for non-shared pixmaps

this allows the pixmap dirty helper to be used for reverse optimus,
where the GPU wants to copy from the shared pixmap to its VRAM copy.

[airlied: slave_dst is wrong name now but pointless ABI churn at this point]
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

fb/fbpixmap.c

@@ -67,6 +67,7 @@ fbCreatePixmapBpp(ScreenPtr pScreen, int width, int height, int depth, int bpp,
     pPixmap->devKind = paddedWidth;
     pPixmap->refcnt = 1;
     pPixmap->devPrivate.ptr = (pointer) ((char *) pPixmap + base + adjust);
+    pPixmap->master_pixmap = NULL;
 
 #ifdef FB_DEBUG
     pPixmap->devPrivate.ptr =