drm/i915/dsi: double check element parsing against size if present

Be a little paranoid in case the specs change or something.

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/27050d48d0ff3d58e79fcacc41ced4c071b01424.1474286487.git.jani.nikula@intel.com