msm: kgsl: Keep consistent values for non-loadable perf counters

PWR, VBIF, and VBIF_PWR performance counters are read-only, and
can't be reloaded with any saved values after they get reset on
device suspend/resume. However the counter read functions can
return consistent values by recalculating them.

Change-Id: I64022ba20899311624635ab6956e8e3cbf172fca
Signed-off-by: Vladimir Razgulin <vrazguli@codeaurora.org>