drm/i915: Handle max_bpc==16

EDID can declare the maximum supported bpc up to 16,
and apparently there are displays that do so. Currently
we assume 12 bpc is tha max. Fix the assumption and
toss in a MISSING_CASE() for any other value we don't
expect to see.

This fixes modesets with a display with EDID max bpc > 12.
Previously any modeset would just silently fail on platforms
that didn't otherwise limit this via the max_bpc property.
In particular we don't add the max_bpc property to HDMI
ports on gmch platforms, and thus we would see the raw
max_bpc coming from the EDID.

I suppose we could already adjust this to also allow 16bpc,
but seeing as no current platform supports that there is
little point.

Cc: stable@vger.kernel.org
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2632
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201110210447.27454-1-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>