drm/ssd130x: fix ssd132x encoding

drm/ssd130x: fix ssd132x encoding

The ssd132x buffer is encoded one pixel per nibble, with two pixels in
each byte.  When encoding an 8-bit greyscale input, take the top 4-bits
as the value and ensure the two pixels are distinct and do not overwrite
each other.

Fixes: fdd591e00a9c ("drm/ssd130x: Add support for the SSD132x OLED controller family")
Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250115110139.1672488-2-jkeeping@inmusicbrands.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>