msm: kgsl: Idle the dispatcher before writing perfcounter select

The GPU must be idle before writing to the perfcounter select
registers to avoid an AHB timeout. Idle the GPU before writing
to the register.

Change-Id: Ic0dedbadf8ad3f10f78ddf68520afe6cf98880e4
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>