msm: kgsl: Consolidate and correctly update reading pwrstats

Only read the various power counters before turning the clocks
off or when reporting to devfreq.  Reading them at other times
is redundant.  Make sure to accumulate the same set of counters
at both points.

Change-Id: I0e4e988d25a18fcede0f0fb022cb7783f5354cd2
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>