vga: Remove some "should be done in BIOS" comments

Not all platforms have a VGA BIOS, powerpc typically relies on
using the DISPI interface to initialize the card.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
