drm/tilcdc: Do not keep vblank interrupts enabled all the time

END_OF_FRAME interrupts have been enabled all the time since the
beginning of this driver. It is about time to add this feature.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fb87adebaffe8c7cb4fe7e909a45d47af08d7c6e.1602349100.git.jsarha@ti.com