msm: kgsl: Do not start performance counters if not present

Certain hardware does not support performance counters and does not set
them.  Ensure the uninitialized performance counter variable does not
get dereferenced in this instance.

Change-Id: I8692090d60ff1e6a0c45b5699b90d9808ef61c5a
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>