drivers: display: mb_display: Unify image and string APIs

Both the string and image rendering may want to take advantage of
scrolling and sequential display capabilities. Consolidate the APIs so
that there's a single one for images (mb_display_image) and a single
one for strings (mb_display_print). Both take a duration parameter for
the per-frame duration as well as a mode parameter which specifies
sequential vs scrolling behavior as well as an optional looping flag.

Change-Id: Ia092d771e3f1b94afd494c7544dab988161c539e
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>