mesa: Throw an error for a new query on an already-active query target.

mesa: Throw an error for a new query on an already-active query target.

There's a similar test below, but it's not the same: that one checks whether
this query object is already active (potentially on another target).

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>