drm/atomic: Introduce helper to lookup connector by encoder

drm/atomic: Introduce helper to lookup connector by encoder

With the bridges switching over to drm_bridge_connector, the direct
association between a bridge driver and its connector was lost.

This is mitigated for atomic bridge drivers by the fact you can access
the encoder, and then call drm_atomic_get_old_connector_for_encoder() or
drm_atomic_get_new_connector_for_encoder() with drm_atomic_state.

This was also made easier by providing drm_atomic_state directly to all
atomic hooks bridges can implement.

However, bridge drivers don't have a way to access drm_atomic_state
outside of the modeset path, like from the hotplug interrupt path or any
interrupt handler.

Let's introduce a function to retrieve the connector currently assigned
to an encoder, without using drm_atomic_state, to make these drivers'
life easier.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Tested-by: Herve Codina <herve.codina@bootlin.com>
Co-developed-by: Simona Vetter <simona.vetter@intel.com>
Signed-off-by: Simona Vetter <simona.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250313-bridge-connector-v6-4-511c54a604fb@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>