ARM: dts: msm: Enable vmem_plus governor on 8996

The current vmem governor does not scale up vmem clock
frequencies up to expected levels. This would result in
frame drops, especially during concurrent video sessions.
The new vmem governor fixes frame drops by ensuring that
the  vmem clock is always scaled one level below the video
core clock.

Change-Id: I04cfea2dbd7ad33e6663088333123106e6dfb51b
Signed-off-by: Arun Menon <avmenon@codeaurora.org>