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
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index ef15f17..10e4716 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -457,7 +457,7 @@
 	u32 swsci_gbda_sub_functions;
 	u32 swsci_sbcb_sub_functions;
 	struct opregion_asle *asle;
-	void *vbt;
+	const void *vbt;
 	u32 *lid_state;
 	struct work_struct asle_work;
 };