drm/i915/dsi: defend gpio table against out of bounds access

Do not blindly trust the VBT data used for indexing.

Cc: stable@vger.kernel.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/cc32d40c2b47f2d2151811855ac2c3dabab1d57d.1454582914.git.jani.nikula@intel.com