[media] vb2: start messages with a lower-case for consistency

The kernel debug messages produced by vb2 started either with a
lower or an upper case character. Switched all to use lower-case
which seemed to be what was used in the majority of the messages.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Pawel Osciak <pawel@osciak.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>