[media] media: davinci_vpfe: use monotonic timestamp

V4L2 drivers should use MONOTONIC timestamps instead of gettimeofday,
which is affected by daylight savings time.

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>