drm/atomic: Convert drm_atomic_get_connector_state() to use new connector state

drm/atomic: Convert drm_atomic_get_connector_state() to use new connector state

The drm_atomic_get_connector_state() function calls a hand-rolled
implementation of the deprecated
drm_atomic_get_existing_connector_state() helper to get find if a
connector state had already been allocated and was part of the given
drm_atomic_state.

At the point in time where drm_atomic_get_connector_state() can be
called (ie, during atomic_check), the existing state is the new state
and drm_atomic_get_existing_connector_state() can thus be replaced by
drm_atomic_get_new_connector_state().

Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com> # on imx8mp
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250930-drm-no-more-existing-state-v5-1-eeb9e1287907@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>