drm/atomic: Convert drm_atomic_get_private_obj_state() to use new plane state

drm/atomic: Convert drm_atomic_get_private_obj_state() to use new plane state

The drm_atomic_get_private_obj_state() function tries to find if a
private_obj had already been allocated and was part of the given
drm_atomic_state. If one is found, it returns the existing state
pointer.

At the point in time where drm_atomic_get_private_obj_state() can be
called (ie, during atomic_check), the existing state is the new state
and we can thus replace the hand-crafted logic by a call to
drm_atomic_get_new_private_obj_state().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250930-drm-no-more-existing-state-v5-38-eeb9e1287907@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>