drm/nouveau/nvfw: firmware structures should begin with nvfw_

Rename all structures that are used directly by firmware to have a nvfw_
prefix.

This makes it easier to identify structures that have a fixed, specific
layout.  A future patch will define several more such structures, so it's
important to be consistent now.

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>