drm/amd/display: Use DAC load detection on analog connectors (v2)

This feature is useful for analog connections without EDID:
- Really old monitors with a VGA connector
- Cheap DVI/VGA adapters that don't connect DDC pins

When a connection is established through DAC load detection,
the driver is supposed to fill in the supported modes for the
display, which we already do in amdgpu_dm_connector_get_modes.

Also, because the load detection causes visible glitches, do not
attempt to poll the connector again after it was detected this
way. Note that it will still be polled after sleep/resume or
when force is enabled, which is okay.

v2:
Add dc_connection_dac_load connection type.
Properly release sink when no display is connected.
Don't print error when EDID isn't read from an analog display.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -7248,6 +7248,16 @@ amdgpu_dm_connector_poll(struct amdgpu_dm_connector *aconnector, bool force)
 	enum dc_connection_type conn_type = dc_connection_none;
 	enum drm_connector_status status = connector_status_disconnected;
 
+	/* When we determined the connection using DAC load detection,
+	 * do NOT poll the connector do detect disconnect because
+	 * that would run DAC load detection again which can cause
+	 * visible visual glitches.
+	 *
+	 * Only allow to poll such a connector again when forcing.
+	 */
+	if (!force && link->local_sink && link->type == dc_connection_dac_load)
+		return connector->status;
+
 	mutex_lock(&aconnector->hpd_lock);
 
 	if (dc_link_detect_connection_type(aconnector->dc_link, &conn_type) &&