graphics/nvidia-drm-[510,515]-kmod: Add the port.

This port provides the nvidia-drm.ko kernel module, as ported here:
https://github.com/amshafer/nvidia-driver

This module implements the DRM API, which is commonly used for PRIME and
Wayland setups.

Differential Revision: https://reviews.freebsd.org/D41497

Reviewed by: arrowd