drm/core: Set legacy_cursor_update in drm_atomic_helper_disable_plane.

legacy_cursor_update was being set in restore_fbdev_mode_atomic which was
probably unintended. Fix this by only setting it in the function that needs it.

This oversight was introduced in

commit bbb1e524
Author: Rob Clark <robdclark@gmail.com>
Date:   Tue Aug 25 15:35:58 2015 -0400

    drm/fb-helper: atomic restore_fbdev_mode()...

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[Jani: checkpatch fix]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447237751-9663-2-git-send-email-maarten.lankhorst@ubuntu.com