msm: kgsl: Return -ENOENT if a context is detached during submit

There are several points during the submission pipeline where we can
bail if the context has been detached. Return -ENOENT in those cases
so the dispatcher can 1) not try to requeue the command or the
context and 2) not log an error (detached commands should be
silently discarded).

Change-Id: Ic0dedbad5d67b24e8bd2804647133d03617ea8dc
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>