Update libdrm-nouveau1 to the ABI of Linux 2.6.34

Drop 03_revert_abi_change.diff.  Bump libdrm-nouveau shlibs and
symbols versions to 2.4.20-3~ to ensure that packages built against
this version are not used with an older libdrm-nouveau1 version.

Add versioned Breaks against older xserver-xorg-video-nouveau packages
which would segfault if used with this version of libdrm.