drm/nouveau/gr/gk20a: use same initialization sequence as nvgpu

GK20A's initialization was based on GK104, but differences exist in the
way the initial context is built and the initialization process itself.

This patch follows the same initialization sequence as nvgpu performs
to avoid bad surprises. Since the register bundles initialization also
differ considerably from GK104, the register packs are now loaded from
firmware files, again similarly to what is done with nvgpu.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>