libdrm: enable omap support

fixes build for xorg.xf86videoomap

pkgs/development/libraries/libdrm/default.nix

···
   mesonFlags = [
     "-Dnm-path=${stdenv.cc.targetPrefix}nm"
     "-Dinstall-test-programs=true"
+    "-Domap=true"
   ] ++ lib.optionals (stdenv.isAarch32 || stdenv.isAarch64) [
     "-Dtegra=true"
     "-Detnaviv=true"