media: mtk-vcodec: fix vp9 content playback error with show exist frame

Update referenced frame buffer's reference count when playing vp9
content which has show_existing_frame flag, and copy enough buffer
data to current shown frame.

Signed-off-by: Maoguang Meng <maoguang.meng@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>