drm/amd/display: Add return value for detect dp.

System soft hang when hotplug specific 4K DP panel
due to link caps read error and incorrect link setting
parmas to enable dp.
Add status check for DPCD read and add return value
for detect dp, in case of false, return from caller,
avoid further false operation.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>