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

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!