msm: kgsl: Remove the emulated PWR counter group

a3xx had a dedicated block of counters that counted (among other things)
the GPU busy ticks for frequency scaling. That block went away in a5xx
but we kept the counter group around anyway and emulated it so that
the rest of the code didn't need to change.

Unfortunately, this won't fly for a6xx since the register we want to
use is up high in the GMU CX region and making it available through
a counter group means that we need to make the register accessible
from the command stream and that breaks register address protection
on some a6xx targets.

It turns out that nobody outside of the kernel really uses the PWR group
anyway, so we can safely remove it and just directly set up the counters
internally and use them.  Remove the power group for a5xx and a6xx and
for a3xx remove PWR:1 from the available pool of counters.  For each
target make the appropriate counter fixed function and always enabled.

Change-Id: Ic0dedbad4a134340f2b4f8db428ba682ee3d4788
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>