media: mtk-vcodec: support single-buffer frames

MT8183 will use a multi-planar format backed by a single buffer. Adapt
the existing code to be able to handle such frames instead of assuming
each frame is backed by two buffers.

Co-developed-by: Alexandre Courbot <acourbot@chromium.org>
[acourbot: refactor, cleanup and split]

Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: fix checkpatch alignment warning]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>