drm/i2c/ch7006: Use helper to turn off CRTC

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

Cc: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/ec7d5c06e2a21dfb8f9fc9413d3e6df86d6aa1fb.1465392124.git.lukas@wunner.de