drm/tilcdc: crtc: Use drm_atomic_helper_check_crtc_primary_plane()

drm/tilcdc: crtc: Use drm_atomic_helper_check_crtc_primary_plane()

In the tilcdc_crtc_atomic_check(), the tilcdc driver hand-crafts its own
implementation of drm_atomic_helper_check_crtc_primary_plane(). And it
does so by accessing the state pointer in drm_atomic_state->planes which
is deprecated.

Let's use the right helper here.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Jyri Sarha <jyri.sarha@iki.fi>
Link: https://lore.kernel.org/r/20250930-drm-no-more-existing-state-v5-7-eeb9e1287907@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>