drm/i915/opregion: make VBT pointer a const

Because we can. It's not to be touched so tell the compiler too.

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/4b1872b121fb34a193cff9a5cb4e7c858d4a55aa.1450089383.git.jani.nikula@intel.com