mesa: have vdpau drivers in lib/vdpau again

This was done by @abbradar 364f297bb2 but I lost that change by accident
when doing large closure-size merges 39ebb01d.
Fortunately it seems to have caused no problems, likely because
${mesa_drivers}/lib gets into $LD_LIBRARY_PATH.

pkgs/development/libraries/mesa/default.nix

···
 
   '' + /* set the default search path for DRI drivers; used e.g. by X server */ ''
     substituteInPlace "$dev/lib/pkgconfig/dri.pc" --replace '$(drivers)' "${driverLink}"
-  '' + /* move vdpau drivers to $drivers/lib, so they are found */ ''
-    mv "$drivers"/lib/vdpau/* "$drivers"/lib/ && rmdir "$drivers"/lib/vdpau
   '';
   #ToDo: @vcunat isn't sure if drirc will be found when in $out/etc/, but it doesn't seem important ATM */