drm/radeon: add missing crtc unlock when setting up the MC

commit f0d7bfb9 upstream.

Need to unlock the crtc after updating the blanking state.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>