msm: kgsl: On release only detach contexts owned by the instance

There can be multiple device instances open for a process. When
a instance is released only destroy the contexts owned by that
instance.

Change-Id: Ic0dedbad818224f01397db8a8f5a83072492dc59
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>