drm/tilcdc: Register cpufreq notifier after we have initialized crtc

Register cpufreq notifier after we have initialized the crtc and
unregister it before we remove the ctrc. Receiving a cpufreq notify
without crtc causes a crash.

Reported-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>