drm/atomic: Verify connector->funcs != NULL when clearing states

Unfortunately since we don't have Dave's connector refcounting patch
here yet, it's very possible that drm_atomic_state_default_clear() could
get called by intel_display_resume() when
intel_dp_mst_destroy_connector() isn't completely finished destroying an
mst connector, but has already finished setting connector->funcs to
NULL. As such, we need to treat the connector like it's already been
destroyed and just skip it, otherwise we'll end up dereferencing a NULL
pointer.

This fix is only required for 4.6 and below. David Airlie's patchseries
for 4.7 to add connector reference counting provides a more proper fix
for this.

Changes since v1:
 - Fix leftover whitespace

Upstream fix: 0552f765 ("drm/i915/mst: use reference counted
connectors. (v3)")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Lyude <cpaul@redhat.com>