drm/i915: move display spinlock init to display code

The gem code has no business accessing i915->display directly.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231102155223.2298316-2-jani.nikula@intel.com