drm: drm_plane_force_disable is not for atomic drivers

This way I can explain why it'll be fine to pass a NULL acquire ctx
here in the next patch.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322215058.8671-4-daniel.vetter@ffwll.ch