bcm: change color chan to RGB24 (thanks p.kosyh)

p.kosyh writes:

Hello!  I finally bought rpi4 4Gb specially for 9front. It seems,
that default bpp of framebuffer is 16.  I changed it to 24 (via
cmdline.txt and config.txt) and found, that rendering is much faster!
(May be due removing overheads in 16->24 conversions?)

But on rpi4 r and b channels are swapped.  So, i changed BGR24 to
RGB24 in bcm/screen.c and now it works fine!

sys/src/9/bcm/screen.c

@@ -139,7 +139,7 @@ screeninit(void)
 		chan = RGB16;
 		break;
 	case 24:
-		chan = BGR24;
+		chan = RGB24;
 		break;
 	case 32:
 		chan = XRGB32;