drm: Nuke drm_atomic_helper_connector_set_property

It's dead code, the core handles all this directly now. This also
allows us to unexport drm_atomic_helper_connector_set_property.

The only special case is nouveau which used one function for both
pre-nv50 legacy modeset code and post-nv50 atomic world instead of 2
vtables. But amounts to exactly the same.

What is rather strange here is how few drivers set this up, I suspect
the earlier patch to handle properties in the core did end up fixing a
pile of possible issues.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: intel-gfx@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170725080122.20548-7-daniel.vetter@ffwll.ch
Acked-by: Vincent Abriou <vincent.abriou@st.com>