drivers: display: mb_display: Get rid of unnecessary image tracking

We don't need to store copies of the current and next image when
scrolling text, since looking up the font is just as efficient and
also consumes less memory.

Change-Id: Ia905164c5b5784afb52cb2bb38c1ab1d00817df0
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>