Use shiny new drm_crtc_force_disable() instead of open coding the same.
No functional change intended.

Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/95bfab0175ea327f777d43dcc18034bf42ca2287.1465392124.git.lukas@wunner.de