drm/nouveau: Fix AGP reset when AGP FW is already enabled on init.

Previously nouveau_mem_reset_agp() was only disabling AGP fast writes
when coming back from suspend. However, the "locked out of the card
because of FW" problem can also be reproduced on init if you
unload/reload nouveau.ko several times. This patch makes the AGP code
reset FW on init.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>