media: vb2: Minor function naming fix

Minor fix for helper function and comment, s/v4l2_vb2_buffer/vb2_v4l2_buffer.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>