V4L/DVB (5971): ivtv-fb: framebuffer timings no longer locked on module load

Framebuffer timings are currently locked to the video format in use when the
module is loaded. If the video format is then changed, the timings returned
by the framebuffer will be for the original format.

This patch ensures that the timings returned reflect the current video format.

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>