drivers/display: mb_display: Use 4 ms refresh timer

The code was so far requesting 5 ms refresh timer, however with a
default TICKS_PER_SECOND=100 this was always rounded up to 10 ms,
causing some flickering. The closest TICKS_PER_SECOND that will give
flicker-less display but also give optimal ms-to-ticks calculations is
250, so change the timer to match that. After this change any code
using the display is recommended to set 250 as ticks per second.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>