msm: kgsl: Prevent device start after it is already started

Check if device is already started in high priority workqueue thread,
this check prevents calling device start after it is already started,
thereby avoiding unpredictable behavior.

Change-Id: Iffcd065f8c02746d168c8e5779d10d4b0e2404a9
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>