msm: kgsl: use kgsl_context_detached() in adreno_dispatcher_queue_cmd()

The CTXT_FLAGS_BEING_DESTROYED isn't a reliable way to check if
a context is detached as it is only set if the context is the active
context while it is being destroyed.

Change-Id: I494dc283a5bd4f68c78a85fbd2aae17961b9b34e
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>