[media] coda: reuse src_bufs in coda_job_ready

The v4l2_m2m_num_src_bufs_ready() function is called in multiple places
in coda_cob_ready, and there already is a variable src_bufs that is
assigned to its result. Move it to the beginning and use it everywhere.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>