drm/tilcdc: Initialize crtc->port

Initialize port device node pointer in the tilcdc crtc. Fixes "Falling
back to first CRTC" warning from tda998x driver.

The tda998x encoder driver calls drm_of_find_possible_crtcs() to
initialize possible_crtcs of struct drm_encoder. The crtc->port needs
to be initialized for drm_of_find_possible_crtcs() to work.

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