[media] media: davinci_vpfe: set minimum required buffers to three

this patch sets nbuffers to three or more and drops the
unset member video_limit which just a copy paste from
earlier driver.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>