Replace 'crtc->state' with the explicit old crtc state.

Actually it shouldn't matter whether we use the old or the new
crtc state here since any plane that has been removed from the
crtc since the crtc state was duplicated will have been added
to the atomic state already. That is, you can't call
drm_atomic_set_crtc_for_plane() without having the new
plane state already in hand.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101184646.14753-1-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Harry Wentland <harry.wentland@amd.com>