drm/i915: Fix wrong initializer for "locked" variable in assert_panel_unlocked

Otherwise it just contains random memory.

Issue detected by cppcheck.

Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>