msm: kgsl: Avoid turning on GPU clock if already off

In adreno_idle(), check if the GPU clock is already
turned off. If so, the GPU is already idle,
so don't turn on the clocks again.

Change-Id: I27ae939253ee71830466e3117bed0230d7ff67d5
Signed-off-by: Ananta Kishore K <akollipa@codeaurora.org>