new port: graphics/drm-stable-kmod

This is a repo copy of the last 4.9 status of graphics/drm-next-kmod. While drm-next-kmod serves as the experimental DRM/KMS port, this new port will always trail it and be in a more stable state. This is not a guarantee that drm-stable-kmod will receive all versions from drm-next-kmod, more likely there will be jumps.

Mark the two ports as mutually conflicting.

Reviewed by: swills (mentor)
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D14584