bcm2835: Dynamically calculate bytes per pixel parameter

brcm,bcm2708-fb device provided by firmware on RPi5 uses
16 bits per pixel, so lets calculate framebuffer bytes
per pixel dynamically based on queried information.

Tested to work for RPi2b v1.2, RPi3b v1.3, RPi4b v1.1,
RPi2 Zero W, RPi5b v1.0.

Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Tested-by: Jens Maus <mail@jens-maus.de>
Tested-by: Darko Alavanja <darko.alavanja@konsulko.com>
Signed-off-by: Ivan T. Ivanov <iivanov@suse.de>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>