drm/nouveau: use dma.max rather than pushbuf size for checking GET validity

Some upcoming G80 DMA changes will depend on this, but it's split out for
bisectibility just in case it causes some unexpected issues.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_dma.c

@@ -130,7 +130,7 @@ READ_GET(struct nouveau_channel *chan, uint32_t *get)
 
 	val = nvchan_rd32(chan, chan->user_get);
 	if (val < chan->pushbuf_base ||
-	    val >= chan->pushbuf_base + chan->pushbuf_bo->bo.mem.size) {
+	    val > chan->pushbuf_base + (chan->dma.max << 2)) {
 		/* meaningless to dma_wait() except to know whether the
 		 * GPU has stalled or not
 		 */