Copy port to x11/libdrm, do not install from wip.

Implements changes to libdrm to work with mesa 17.0.1 discussed in
pkg/51795
(x11/libdrm update xf86drm.c for mesa 17.0.1 needs negotiation)

Figure out why NetBSD prefers the incorrect version of
atomic_add_unless that returns the c == unless comparison.