media: mediatek: vcodec: add encoder power management helper functions

Remove PM functions at start/stop streaming, add PM helper functions
to get PM before encoding frame start and put PM after encoding frame
done. Meanwhile, remove unnecessary clock operations.

Signed-off-by: Irui Wang <irui.wang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>