media: videobuf2: Print videobuf2 buffer state by name

For debugging purposes, seeing the state integer
representation is really inconvenient.

Improve this and be developer-friendly by printing
the state name instead.

Suggested-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>