drm/i915: constify bxt stepping info

Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445344713-1407-2-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_csr.c

@@ -172,7 +172,7 @@ static const struct stepping_info skl_stepping_info[] = {
 	{'G', '0'}, {'H', '0'}, {'I', '0'}
 };
 
-static struct stepping_info bxt_stepping_info[] = {
+static const struct stepping_info bxt_stepping_info[] = {
 	{'A', '0'}, {'A', '1'}, {'A', '2'},
 	{'B', '0'}, {'B', '1'}, {'B', '2'}
 };