No video output when connect display through WD19 TBT dock on the TGL platform's tbt port.

There is no video output when system is ONLY connected to the WD19TB video output port and WD19TB is connected to the TBT port on the TGL system.

Note:

1. There is no other monitor connect to the built-in DP port.
2. If remove the intel_dp_lttpr_init from intel_dp_get_dpcd, the machine can work well with TBT dock.
3. Test the last drm-tip(20201215) and 5.6 kernel with the backported LTTPR patches, the error message would be prompted. "> [drm:drm_dp_dpcd_access [drm_kms_helper]] Too many retries, giving up. First error: -71"

[Reproduce Steps]

1. Connect WD19TB to the TBT port on the TGL desktop platform
2. Connect a monitor to video output port on the WD19TB. (There is no other monitor connected to the TGL desktop platform)
3. Boot up system
4. observe there is no video output on the display