media: mediatek: vcodec: Add a debugfs file to get different useful information

In oder to get each instance information according to test command, adding
one file node "vdec".

Can use echo command to set different string value as
'echo -picinfo > vdec'.

Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: drop unwanted dbgfs_inst NULL ptr check in mtk_vcodec_dbgfs_remove]
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>