display: ssd1673: replace hardcoded display dimensions by DT values

Instead of hardcoding multiple times the display dimensions, use the
values from DT. This still assume 8 rows per page and 8 pixels per
bytes, but that should always be the case for this controller and a
monochrome display.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>