drm/amd/display: Skip Check Runtime Link Setting for Specific Branch Device

[why]
Read link setting inside mode validation is not always the final downlink setting.
It is found true in Synaptics branch device.

At bootup, the preferred mode being set right after 1080p is set. It occurred
before graphic load. That modeset switch in a short period of time makes
the branch device switch back and forth from lower and higher link rate,
observed at Synaptics branch device.
DP2 RTK hub on the other hand, sticks to highest available downlink rate after bootup.

Existing check of runtime downlink setting in mode validation shows asynchronous at
branch device link switch, i.e., downlink switch to higher link rate not yet complete
when the mode validation tries to probe the downlink setting. That makes mode validation
checking downlink setting making wrong decision by pruning modes that should pass the
validation after the downlink setting switch is complete.

[how]
If Synaptics is found at the last branch, skip checking downlink setting
at mode validation.

Reviewed-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -768,6 +768,9 @@ struct amdgpu_dm_connector {
 	uint16_t vc_full_pbn;
 	struct mutex handle_mst_msg_ready;
 
+	/* branch device specific data */
+	uint32_t branch_ieee_oui;
+
 	/* TODO see if we can merge with ddc_bus or make a dm_connector */
 	struct amdgpu_i2c_adapter *i2c;