[media] vb2: use correct prefix

Many dprintk's in vb2 use a hardcoded prefix with the function name. In
many cases that is now outdated. To keep things consistent the dprintk
macro has been changed to print the function name in addition to the "vb2:"
prefix. Superfluous prefixes elsewhere in the code have been removed.

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>