drm/i915/gt: add a macro for mock gt wakeref special value and use it

drm/i915/gt: add a macro for mock gt wakeref special value and use it

Add a dedicated macro for the special mock gt wakeref value, with a cast
to intel_wakeref_t, instead of assuming you can assign or compare the
wakeref to -ENODEV directly.

Arguably the whole thing is a hack that should not exist, but at least
make it slightly less hacky.

Side note: If this value were to ever end up in
intel_ref_tracker_free(), it would wreak havoc.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1da887a6b4fe1ec45355571ea7b56d91fadf0af2.1726680898.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>