drm/atomic: Remove unused drm_atomic_get_existing_connector_state()
The drm_atomic_get_existing_connector_state() function is deprecated and
isn't used anymore, so let's remove it.
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250930-drm-no-more-existing-state-v5-2-eeb9e1287907@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>