drm: Add drm_atomic_get_(old|new)_connector_for_encoder() helpers

Add functions to the atomic core to retrieve the old and new connectors
associated with an encoder in a drm_atomic_state. This is useful for
encoders and bridges that need to access the connector, for instance for
the drm_display_info.

The CRTC associated with the encoder can also be retrieved through the
connector state, and from it, the old and new CRTC states.

Changed in v4:
- Added to the set
Changed in v5:
- Fix up docbook (Daniel & Laurent)
Changed in v6:
- Updated commit subject (Sam)

Link to v4: https://patchwork.freedesktop.org/patch/msgid/20190508160920.144739-3-sean@poorly.run
Link to v5: https://patchwork.freedesktop.org/patch/msgid/20190611160844.257498-3-sean@poorly.run



Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

[seanpaul removed WARNs from helpers and added docs to explain why
returning NULL might be valid]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611205147.181298-1-sean@poorly.run