drm/i915: move display mutex inits to display code

The core code has no business accessing i915->display directly. These
could be further spread to respective files, but this is a start.

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/20231102155223.2298316-1-jani.nikula@intel.com