msm: kgsl: Prevent adreno stop after gpu is power collapsed

When GPU is power collapsed GPU is already stopped. If kgsl release
gets called do not try to stop the GPU again. Trying to stop
already stopped GPU can lead to errors.

When content protection is enabled we cannot write to VBIF
registers with iommu detached. With this limitation if
adreno stop gets called twice, the second adreno stop will
cause NOC errors/XPU violations because trustzone will
XPU lock down all VBIF registers after first adreno stop.

Prevent adreno stop getting called twice by checking if device
is started, only if device is started go ahead with adreno
stop.

CRs-fixed: 726670
Change-Id: I4e3c7a9b37eb88d458d65763ed6818a4fd96bd06
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>