Implement polling for VGA and SIL164 connectors. Set the flag
DRM_CONNECTOR_POLL_DISCONNECT for each to detect the removal of the
monitor cable. Implement struct drm_connector_helper_funcs.detect_ctx
for each type of connector by testing for EDID data.

The helper drm_connector_helper_detect_ctx() implements .detect_ctx()
on top of the connector's DDC channel. The function can be used by
other drivers as companion to drm_connector_helper_get_modes().

v6:
- change helper name to drm_connector_helper_detec_from_ddc()
  (Maxime, Sui)
v5:
- share implementation in drm_connector_helper_detect_ctx() (Maxime)
- test for DDC presence with drm_probe_ddc() (Maxime, Jani)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240325200855.21150-13-tzimmermann@suse.de