drm/omap: omap_gem: Do not try to unmap page which is not mapped

It might be possible that the page has been unmapped already in
omap_gem_cpu_sync() so check before calling dma_unmap_page().
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/omapdrm/omap_gem.c

@@ -336,7 +336,9 @@ static void omap_gem_detach_pages(struct drm_gem_object *obj)
 	if (omap_obj->flags & (OMAP_BO_WC|OMAP_BO_UNCACHED)) {
 		int i, npages = obj->size >> PAGE_SHIFT;
 		for (i = 0; i < npages; i++) {
-			dma_unmap_page(obj->dev->dev, omap_obj->addrs[i],
+			if (omap_obj->addrs[i])
+				dma_unmap_page(obj->dev->dev,
+					       omap_obj->addrs[i],
 					       PAGE_SIZE, DMA_BIDIRECTIONAL);
 		}
 	}