drm/nouveau/mcp89/mmu: Use mcp77_mmu_new instead of g84_mmu_new on MCP89.

Fix a crash or broken depth testing in all OpenGL applications that use the
depth buffer on MCP89 (GeForce 320M) seen on a MacBook Pro Late 2010.

The bug is tracked in https://bugs.freedesktop.org/show_bug.cgi?id=108500

Signed-off-by: Timo Wiren <timo.wiren@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>