drm/i915/gt: Hold a wakeref for the active VM

There may be a disconnect between the GT used by the engine and the GT
used for the VM, requiring us to hold a wakeref on both while the GPU is
active with this request.

v2: added explanation to __queue_and_release_pm

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[ahajda: removed not-yet-upstremed wakeref tracking bits]
Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230330-hold_wakeref_for_active_vm-v2-1-724d201499c2@intel.com