drivers: display: mb_display: Make display context more compact

Reorder the struct members for more compact layout. The current row
tracker tracks just values 0 to 2 so uint8_t is more than enough for
it.

Change-Id: I845c84aeb32d59ed0ebbd55d8b6cfda7ad19b75a
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>