drm/tilcdc: Call drm_crtc_vblank_on() and *_off() in start() and stop()

Add drm_crtc_vblank_on() and *_off() calls to start() and stop()
functions, to make sure any vblank waits etc. gets properly cleaned
up.

Signed-off-by: Jyri Sarha <jsarha@ti.com>