msm: kgsl: Don't wait for context that has not submitted

A context may be detached without submitting any commands
to GPU ringbuffer. This may cause us to wait on a timestamp
that will never be retired. So return immediately from
adreno_drawctxt_wait_rb() if context has not submitted any
commands to GPU ringbuffer.

Change-Id: If8b3f8df92ec9b54a1a83d2f6704d4d15eb1b979
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>