drm/i915: cleanup_plane_fb: also drop reference to current state wait_req

drm/i915: cleanup_plane_fb: also drop reference to current state wait_req

There are two paths into intel_cleanup_plane_fb, the normal completion
path and the failure path.

In the failure case, intel_cleanup_plane_fb is called before
drm_atomic_helper_swap_state, so any wait_req reference made in
intel_prepare_plane_fb will be in old_intel_state->wait_req.

In the normal completion path, drm_atomic_helper_swap_state has
already been called, so the plane state holding the just-used wait_req
will not be in old_intel_state->wait_req, rather it will be in the
state associated with the plane itself.

Clearing this reference ensures that the wait_req will be freed as
soon as it the related mode setting operation is complete, rather than
waiting for some future mode setting operation to eventually
dereference it.

The existing dereference of old_intel_state->wait_req is still
required as that will hold the wait_req when the mode setting
operation fails.

cc: Daniel Vetter <daniel.vetter@intel.com>
cc: David Airlie <airlied@linux.ie>
cc: intel-gfx@lists.freedesktop.org
cc: dri-devel@lists.freedesktop.org
Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>