drm/loongson: Switch to drm_atomic_get_new_crtc_state()

drm/loongson: Switch to drm_atomic_get_new_crtc_state()

The loongson atomic_check implementation uses the deprecated
drm_atomic_get_existing_crtc_state() helper.

This hook is called as part of the global atomic_check, thus before the
states are swapped. The existing state thus points to the new state, and
we can use drm_atomic_get_new_crtc_state() instead.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250930-drm-no-more-existing-state-v5-23-eeb9e1287907@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>