msm: kgsl: Put a keep alive vote before updating CP_RB_WPTR

There could be a situation where GPU comes out of ifpc after a
fenced write transaction but before reading AHB_FENCE_STATUS from
KMD, it goes back to ifpc due to inactivity.
Put a keep alive vote before doing fenced write for CP_RB_WPTR to
avoid such unlikely scenario.

Change-Id: Ibe0391b39cedf64e2dfa78ab0bbcda0fc8746c88
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>