drm: Add helpers to turn off CRTCs

Turning off a single CRTC or all active CRTCs of a DRM device is a
fairly common pattern. Add helpers to avoid open coding this everywhere.

The name was chosen to be consistent with drm_plane_force_disable().

Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>