msm: kgsl: Do not allow a fence to be created on an invalid context

Don't allow a new fence to be created on an invalidated context thus
preventing fence timeouts for timestamps that are never going to
happen.

Change-Id: Ic0dedbad3b9e47f14f0bdce748159c1ddd8c1a26
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>