drm/nv04-nv3x: Implement init-compute-mem.

Init-compute-mem was the last piece missing for nv0x-nv3x card
cold-booting. This implementation is somewhat lacking but it's been
reported to work on most chipsets it was tested in. Let me know if it
breaks suspend to RAM for you.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Tested-by: Patrice Mandin <patmandin@gmail.com>
Tested-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Xavier Chantry <chantry.xavier@gmail.com>
Tested-by: Marcin Kościelnicki <koriakin@0x04.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>