drivers: display: mb_display: Add internal reset helper

Add a helper function to reset the state of the display, and make use
of it from the various public functions. This also ensures that the
timer is stopped before starting a new display routine.

Change-Id: I8a916b5b13c18b41b7fc3593e6d97e874ef117af
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>