drm/i915: Release temporary load-detect state upon switching

After we call drm_atomic_commit() on the load-detect state, we can free
our local reference. Upon restore, we only apply and free the previous state.

Fixes: 0853695c

 ("drm: Add reference counting to drm_atomic_state")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: <drm-intel-fixes@lists.freedesktop.org> # v4.10-rc1+
Link: http://patchwork.freedesktop.org/patch/msgid/20170119113749.2517-1-chris@chris-wilson.co.uk
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>