Log Message:
Update to libdrm-2.3.1.  There is no up-to-date ChangeLog in the package
whatsoever, so I cannot include a list here (see GIT history if
you're curious).

NOTE: This version of libdrm no longer includes the TTM interface.
      If any [xorg/mesa] packages are updated to versions explicitly requiring
      libdrm-2.3.1, then BUILDLINK_API_DEPENDS for this package will
      need to be changed.  Also, the i915tex dri driver found in
      graphics/MesaLib will not build with this version--but nor was it
      supported on any platform except Linux.  I have remove i915tex
      from the MesaLib driver build; this should be enough to avoid problems.