Manual merge of OpenBSD -current changes, Xenocara libdrm part, with:

git diff dc120473~..dc120473 openbsd/xenocara/lib/libdrm | \
    patch -d xsrc/external/mit/libdrm/dist -p5
Make #if conditions behave for NetBSD as they do for OpenBSD.
OpenBSD Xenocara's libdrm has been updated to 2.4.31.
Use the same DRM_MAJOR definition as in vanilla NetBSD.