[media] vb2: Rename confusingly named internal buffer preparation functions

Rename __qbuf_*() functions which are specific to a buffer type as
__prepare_*() which matches with what they do. The naming was there for
historical reasons; the purpose of the functions was changed without
renaming them.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>