drm/gma500: fix things after get/put page helpers

Commit 8b9ba7a3 'drm/gma500: use gem get/put page helpers' was missing a
line, which resulted in garbled screen.
Signed-off-by: Rob Clark <robdclark@gmail.com>
Tested-by: Guillaume CLÉMENT <gclement@baobob.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/gma500/gtt.c

@@ -204,6 +204,7 @@ static int psb_gtt_attach_pages(struct gtt_range *gt)
 	if (IS_ERR(pages))
 		return PTR_ERR(pages);
 
+	gt->npage = gt->gem.size / PAGE_SIZE;
 	gt->pages = pages;
 
 	return 0;