drm: Use helper to turn off CRTC

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

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/fce52e55fa5a3a2c1436205d26fc1cec70d01eea.1465392124.git.lukas@wunner.de