msm: kgsl: Add cache flush for A20x

Adding extra cache flush command to make sure that CP block
gets event from RB all the time to avoid race conditions because of
clock switches. this is to improve stability on 8x25 target.

(cherry picked from commit fd5f8a4e3dc902d32f5b37de67555e56b887f759)

Change-Id: I32063b1d4ab6180159f868b47c7e0fd79d7b12b3
Signed-off-by: Gopal Goberu <ggober@codeaurora.org>