[media] ivtvfb: Add sanity check to ivtvfb_pan_display()

Add sanity check to ivtvfb_pan_display() to ensure only valid values are used
to pan the display. Invalid values are rejected with -EINVAL

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>