drivers: display: ssd1322: fix only part of the image being diplayed

Only the first chunk of the desired image is displayed repeatedly over
the screen height (at least on an NHD-2.7-12864WDW3 display). Fix it
by computing the internal mono01 to 4bit grayscale conversion buffer
size correctly so it can fit the entire image.

Signed-off-by: Johan Lafon <johan.lafon@syslinbit.com>