The EDID parser will zero out the bpc value, and the driver needs to handle
that case.  In our picker, we'll just ignore 0 values as far as bpp
picking goes.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=39323.
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>