drm/i915/display: add intel_display_run_as_guest()

drm/i915/display: add intel_display_run_as_guest()

Add intel_display_utils.c for display utilities that need more than a
header.

Start off with intel_display_run_as_guest(). The implementation is
intentional duplication of the i915_utils.h i915_run_as_guest(), with
the idea that it's small enough to not matter.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://patch.msgid.link/469f9c41e0c3e3099314a3cf1a7671bf36ec8ffd.1761146196.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>