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

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

Note:

1. There is no other monitor connect to the built-in DP port.
2. Test the drm-tip(20210118), issue still can be reproduced. the error would show up. #[ 35.703718] i915 0000:00:02.0: [drm] ERROR CPU pipe B FIFO underrun
   here's the drmtip log, wd19tb is conectted with TGL platform, drm_drmtip20200118_log_20200122

[Reproduce Steps]

1. Connect WD19TB to the Type-c 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