drm/i915: Reject panel_type > 0xf from VBT

VBT can only contain 16 panel entries, indexed with the panel_type.
To play it safe we should reject panel_type > 0xf, so that we don't
read past the valid data.

v2: Add debug logging (Jani)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rob Kramer <rob@solution-space.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1460359329-10817-1-git-send-email-ville.syrjala@linux.intel.com