msm: vidc: Add a new governor for vmem

The present vmem governor does not scale the vmem clock
correctly in accordance with the video core clock. Typically,
the vmem clock should always be scaled at one level below the
video core clock frequency. The vmem_plus governor achieves
this scaling for vmem clock.

Change-Id: I7e150631abfbbd2846237c4b197ccca37d09b0f4
Signed-off-by: Arun Menon <avmenon@codeaurora.org>