drm/nouveau/secboot/gm20b,gp10b: Read WPR configuration from GPU registers

The GPUs found on Tegra SoCs have registers that can be used to read the
WPR configuration. Use these registers instead of reaching into the
memory controller's register space to read the same information.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>