st/va/vp9: set max reference as default of VP9 reference number

If there is no information about number of render targets

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Boyuan Zhang <boyuan.zhang@amd.com>
Cc: 19.0 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit a0a52a03)