msm: venc: set perf mode for HEIC buffers

1. Set perf mode for image session buffers so that
they will be processed quickly.
2. Set max frequency for any turbo sessions.

Change-Id: Icf9b0af5cffcd395356783a9ff7a9421a309f52c
Signed-off-by: Shi Zhongbo <zhongbos@codeaurora.org>