msm: kgsl: Keep the context alive until its fences signal

The context is currently kept alive until all its fences are
released. This means that we do not destroy a context until all
the fence fds associated with it are closed by userspace, which may
be a long time. Instead, remove the timeline's refcount on its
context when it is detached. This permits the context to be freed
once all its fences signal.

Change-Id: Ifc5dd55980358ddbb4d3f6220d8b5d9f725aed1b
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>