drm/atomic-helper: Drop unneeded argument from check_pending_encoder

Just a remnant from an old iteration of this patch that I've forgotten
to remove: We only need the encoder to figure out whether it has been
reassigned in this update already or not to figure out whether there's
a conflict or not.

Reported-by: Thierry Reding <thierry.reding@gmail.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449564561-3896-2-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>