msm: kgsl: Overwrite the existing snapshot only if GMU is enabled

For a GPU fault, we always want the first snapshot to debug it,
whether the GPU recovers or not. Don't overwrite an existing
GPU fault snapshot if GMU is not enabled or supported.

Change-Id: I305fb1f190a95d624a0b6379b18bf51e3df2b470
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>