[Why]
If eDP ver >= 1.4,
the Source device must use LINK_RATE_SET.

[How]
Get LINK_RATE_SET by reading DPCD 10h-1fh,
then write DPCD 00115h before link training.
Signed-off-by: Derek Lai <Derek.Lai@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>