media: vivid: Improve timestamping

Simulate a more precise timestamp by calculating it based on the
current framerate.

Signed-off-by: Gabriel Francisco Mandaji <gfmandaji@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: replaced division by 2 with bit shift]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>