drm/amdgpu: Fix connector atomic_check compilation fail

I missed amdgpu in my connnector_helper_funcs->atomic_check conversion,
which is understandably causing compilation failures.

Fixes: 6f3b6278

 ("drm: Convert connector_helper_funcs->atomic_check to accept drm_atomic_state")
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com> [for rcar lvds]
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Karol Herbst <karolherbst@gmail.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: dri-devel@lists.freedesktop.org
Cc: intel-gfx@lists.freedesktop.org
Cc: linux-renesas-soc@vger.kernel.org
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614002713.141340-1-sean@poorly.run