drm/nouveau: do not leak in nv20_graph_create

If we return due to an unknown chipset in
drivers/gpu/drm/nouveau/nv20_graph.c:nv20_graph_create() we'll leak the
memory allocated to 'pgraph'.

This patch should fix the leak.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Reviewed-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>