msm: kgsl: Check GPMU presence before accessing power counters

Some targets do not have GPMU block in GPU and register read
or write into those registers is not valid.

To prevent this, check GPMU presence and then proceed.

Change-Id: I0992125af8cda4b8235aedb64f6ef8868ae95f20
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>