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.

Change-Id: Ie411be46fa166864073d35c50085a8f2506d0eb8
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>