msm: kgsl: Account for fixed perfcounter groups

Most performance counter groups have general purpose counters that
can be programmed to any number of countable types. That said
some groups have counters with fixed countables. When reserving those
groups the countable is equal to the register - so when you ask for
countable 0 in VBIF_PWR for example it is expected that will always
match to a certain register. Add a flag member to the perfcounter
group and declare PWR and VBIF_PWR as fixed groups and handle them
accordingly in adreno_perfcounter_get() so that the correct register
is returned.

CRs-fixed: 582739
Change-Id: Ic0dedbad271c7472efce5f79e9055b7b3d224d17
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>