msm: kgsl: Restructure performance counters

Restructure the performance counter mechanism by combining the two
structures declared for it. The 2 structures can only refer to a
single counter hence it is easier to manage by having them
under a single structure.

Change-Id: I19d13cf5aa619b85a332b383b464c2af65ad38c9
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>