drm/sti: fix atomic_disable check

When a drm_plane is being disabled, its ->crtc member is set to NULL
before the .atomic_disable() func is called.
To get the crtc of the plane, read old_state->crtc instead of
drm_plane->crtc

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>