drm: Make drm_modeset_lock_crtc internal

This is only for legacy paths that need to grab the crtc/plane lock
combo. If you want to lock a crtc, just use drm_modeset_lock().

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-2-daniel.vetter@ffwll.ch