drm/nv40: make INIT_COMPUTE_MEM a NOP, just like nv50

It appears we aren't required to do memory sizing ourselves on nv40
either.  NV40 init tables read a strap from PEXTDEV_BOOT_0 into a
CRTC register, and then later use that value to select a memory
configuration (written to PFB_CFG0, just like INIT_COMPUTE_MEM on
earlier cards) with INIT_IO_RESTRICT_PROG.

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