macfb: fix 24-bit RGB pixel encoding

macfb: fix 24-bit RGB pixel encoding

According to Apple Technical Note HW26: "Macintosh Quadra Built-In Video" the
in-built framebuffer encodes each 24-bit pixel into 4 bytes. Adjust the 24-bit
RGB pixel encoding accordingly which agrees with the encoding expected by MacOS
when changing into 24-bit colour mode.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20211007221253.29024-12-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>