drm/i915/opregion: make VBT pointer a const

Because we can. It's not to be touched so tell the compiler too.
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/4b1872b121fb34a193cff9a5cb4e7c858d4a55aa.1450089383.git.jani.nikula@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -457,7 +457,7 @@ struct intel_opregion {
 	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;
 };

drivers/gpu/drm/i915/intel_opregion.c

@@ -987,7 +987,7 @@ int intel_opregion_setup(struct drm_device *dev)
 		DRM_DEBUG_DRIVER("ASLE extension supported\n");
 
 	if (!dmi_check_system(intel_no_opregion_vbt)) {
-		void *vbt = base + OPREGION_VBT_OFFSET;
+		const void *vbt = base + OPREGION_VBT_OFFSET;
 		u32 vbt_size = OPREGION_ASLE_EXT_OFFSET - OPREGION_VBT_OFFSET;
 
 		if (intel_bios_is_valid_vbt(vbt, vbt_size)) {