drm/i915/syncmap: squelch a sparse warning

The code is fine, really, but tweak it to get rid of the sparse warning:

drivers/gpu/drm/i915/selftests/i915_syncmap.c:80:54: warning: dubious: x | !y

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231129173506.1194437-1-jani.nikula@intel.com