drm/tilcdc: Take CRTC lock when calling tilcdc_crtc_disable()

Take CRTC lock when calling tilcdc_crtc_disable() in
tilcdc_crtc_destroy().

In theory there could still be some operation ongoing, which should
finish before destroying the CRTC. However, the main reason for
adding this is to be able to add WARNing in tilcdc_crtc_disable() if
CRTC is not locked.

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