targets/xorg: Downgrade some extra packages

mesa also provides some other packages (e.g. -dev and -dbg packages),
make sure we also downgrade the ones that break when we try to
install them.

See http://packages.ubuntu.com/source/precise/mesa for a full list,
a number of them probably work fine with more recent GL libraries
and may not need downgrading (e.g. libgbm), so we do not downgrade
them.
Merge pull request #922 from drinkcat/mesa-add-missing-pin

targets/xorg: Downgrade some extra packages