V4L/DVB (12134): vivi: bug: don't assume that S_STD will be called before streaming

precalculate_bars() improved vivi performance. However, it assumed that
always before streaming, the driver would call VIDIOC_S_STD. This is not
an API requirement, and the testing apps don't do that.

Due to that, a regression were caused by the patch that added it.

This patch moves the precalculate_bars to the proper place of the code,
calling it at buffer_prepare() callback.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>