drm/i915: don't use uninitialized EDID bpc values when picking pipe bpp

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>