drm/xe/compat: reduce i915_utils.[ch]

drm/xe/compat: reduce i915_utils.[ch]

Reduce the compat i915_utils.h to only cover the requirements of
MISSING_CASE() in soc/ and the few i915_inject_probe_failure()
instances, instead of including the entire i915_utils.h from i915. This
prevents new users for the utilities from cropping up.

With this, we can remove the xe/display/ext/i915_utils.c altogether.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://patch.msgid.link/c02f82e45f31caf95bd1339080b9099c3b7190be.1761146196.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>