Tang nano 20k hdmi fix

[trabucayre]

This PR fixes two issues with sipeed_tang_nano_20.py target related to HDMI:

• unlike some others sipeed boards, pairs used for HDMI are true differential. As mentioned in soc/cores/video.py: VideoGowinHDMIPHY: allows to specify if _p/_n are true LVDS or if LVDS must be emulated enjoy-digital/litex#2341, a TLVDS_OBUF must be used otherwise apicula fails. the above mentioned PR has introduces the true_lvds CTOR's parameter for VideoGowinHDMIPHY to select between primitives. The first commit simply sets this parameter to True to uses the good primitive
• Second commit is related to add_video_colorbars: HDMI ClockDomain, at CRG level, is configured to have a pix_clk == 25MHz, but timings parameter is fixed to 800x600@60Hz requiring a 40MHz. By switching timings to 640x480@60Hz the colorbars mode is now working.

[enjoy-digital]

Thanks @trabucayre!