drm/via: Add new condition to via_dma_cleanup()

Pointer dev_priv->mmio, which was checked for NULL at via_do_init_map(),
is passed to via_do_cleanup_map() and is dereferenced there without check.

The patch adds the condition in via_dma_cleanup() which prevents potential NULL
pointer dereference.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 22f579c621e2 ("drm: Add via unichrome support")
Signed-off-by: Alisa Khabibrakhmanova <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/via/via_dri1.c

···
 		drm_via_private_t *dev_priv =
 		    (drm_via_private_t *) dev->dev_private;
 
-		if (dev_priv->ring.virtual_start) {
+		if (dev_priv->ring.virtual_start && dev_priv->mmio) {
 			via_cmdbuf_reset(dev_priv);
 
 			drm_legacy_ioremapfree(&dev_priv->ring.map, dev);