drm/atomic: Convert __drm_atomic_get_current_plane_state() to modern accessor

The __drm_atomic_get_current_plane_state() function tries to get and
return the existing plane state, and if it doesn't exist returns the one
stored in the drm_plane->state field.

Using the current nomenclature, it tries to get the existing plane state
with an ad-hoc implementation of drm_atomic_get_existing_plane_state(),
and falls back to either the old or new plane state, depending on
whether it is called before or after drm_atomic_helper_swap_state().

The existing plane state itself is deprecated, because it also changes
when swapping states from the new state to the old state.

Fortunately for us, we can simplify things. Indeed,
__drm_atomic_get_current_plane_state() is only used in two macros:
intel_atomic_crtc_state_for_each_plane_state and
drm_atomic_crtc_state_for_each_plane_state().

The intel variant is only used through the intel_wm_compute() function
that is only ever called in intel_crtc_atomic_check().

The generic variant is more widely used, and can be found in the malidp,
msm, tegra and vc4 drivers. All of these call sites though are during
atomic_check(), so we end up in the same situation than Intel's.

Thus, we only ever use the existing state as the new state, and
plane->state is always going to be the old state. Any plane isn't
guaranteed to be part of the state though, so we can't rely on
drm_atomic_get_old_plane_state() and we still need to use plane->state.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250930-drm-no-more-existing-state-v5-4-eeb9e1287907@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>

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