drm/tilcdc: Enable and disable interrupts in crtc start() and stop()

Enable and disable interrupts in crtc start() and stop(). None of the
interrupts can fire if CRTC is disabled, so it is cleaner - when
considering suspend/resume code etc. - to enable the interrupts when
CRTC is turned on and to disable them when CRTC is turned off.

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