drm/tilcdc: Fix module unloading

Force crtc dpms off before destroying the crtc instead of just
checking the dpms state. This fixes warning message and frozen picture
after tilcdc module unloading.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>