fbdev: Make pixel_to_pat() failure mode more friendly

If we accidentally pass an incorrect bpp value to pixel_to_pat(),
it panics. This is pretty useless, as we generally have the various
console locks held at that point, so nothing will be displayed,
and there is no reason to make this a fatal event.

Let's WARN instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>