msm: kgsl: Do not BUG in GPU recovery if a valid context can't be found

Don't BUG() if we can't find a valid context to recover to; just fail and
go into a zombie state. This usually happens after a first hang goes bad
and the system keeps hanging while trying to find its way. The problem with
a BUG() in this case is that the system goes down and we can't recover the
snapshot or other debug information.