media: videobuf2-core: Rework and rename helper for request buffer count

The helper indicating whether buffers are associated with the request is
reworked and renamed to return the number of associated buffer objects.

This is useful for drivers that need to check how many buffers are in
the request to validate it.

Existing users of the helper don't need particular adaptation since the
meaning of zero/non-zero remains consistent.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>