drm/nouveau: Reset AGP before running the init scripts.

BIOS scripts usually make an attempt to reset the AGP controller,
however on some nv4x cards doing it properly involves switching FW off
and on: if we do that without updating the AGP bridge settings
accordingly (e.g. with the corresponding calls to agp_enable()) we
will be locking ourselves out of the card MMIO space. Do it from
nouveau_mem_reset_agp() before the init scripts are executed.

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