video: fbdev: use msecs_to_jiffies for time conversions

This is only an API consolidation and should make things more readable by
replacing  var * HZ / 1000  by msecs_to_jiffies(var).

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>