msm: vidc: Set fps to 1 for image encode sessions

Framework may, incorrectly, set fps to number of output tiles. As a
result, MBs per second for HEIF encode can be pretty large for high
resolution images. This change sets fps to 1 for image session with
tiling.
Disable power saving for all image sessions.

Change-Id: I2ef8f638421460dc7f6b1f3b3709d0f1a9c5a705
Signed-off-by: Shi Zhongbo <zhongbos@codeaurora.org>