drm/amd/display: verify lane status before exiting verify link cap

[why]
DP LL CTS1.4 4.3.2.1 test failure.

[how]
The failure is caused by not handling DP link loss
hpd short pusle during set mode. The change is to read link status
before set mode link training. If link is lost, re-verify link caps.
Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>