drm/i915/bios: use a flag for vbt hdmi level shift presence

The pre-initialized magic value is a bit silly, switch to a flag
instead.

v2: Reduce paranoia to a single sanity check (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/74fe24ab6d5f0ea2ff2059cdf044d6d3006080fc.1573227240.git.jani.nikula@intel.com