Pull up following revision(s) (requested by riastradh in ticket #1116):
	external/mit/libdrm/dist/xf86atomic.h: revision 1.4
Fix libdrm's atomic_dec_and_test on NetBSD (and Solaris, while here).
_nv means new value, not old value!  This may help to fix various
instability in userland graphics, now that we aren't freeing stuff
with a reference count of 1 any more.