MGS-6371 [#imx-2489] Fixed the gckHARDWARE_SetFscaleValue().

We have to use the commitMutex to ensure when we set frequency scaling,
so the other commits will be stopped.
If we use the gckCOMMAND_Stall + gckHARDWARE_QueryIdle, we might get new commit
between wait stall signal and gckHARDWARE_QueryIdle and we may never get the GPU idle.

Signed-off-by: Wang Zijian <Zijian.Wang@verisilicon.com>
Signed-off-by: Nicusor Citu <nicusor.citu@nxp.com>