nv30: Fix "array subscript is below array bounds" compiler warning

gcc6 does not like the trick where we point to one entry before the
array start and then start a while with a pre-increment.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/nv30/nv30_transfer.c

@@ -663,8 +663,7 @@ nv30_transfer_rect(struct nv30_context *nv30, enum nv30_transfer_filter filter,
       {}
    };
 
-   method = methods - 1;
-   while ((++method)->possible) {
+   for (method = methods; method->possible; method++) {
       if (method->possible(nv30, filter, src, dst)) {
          method->execute(nv30, filter, src, dst);
          return;