drm/i915/display: add lock while printing frontbuffer tracking bits to debugfs

Add missing spin_lock/unlock

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230814163452.2925-1-juhapekka.heikkila@gmail.com