drm/xe/display: remove unused compat kdev_to_i915() and pdev_to_i915()

The display code no longer uses kdev_to_i915() or pdev_to_i915()
helpers. Remove them.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/05b948f9012fc7c0b97d567c70b0bac8791d554a.1724942754.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>