drm/amdgpu: Don't link train DisplayPort on HPD until we get the dpcd

This is a port of:
DRM - radeon: Don't link train DisplayPort on HPD until we get the dpcd
to amdgpu.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

diff --cc drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
Simple merge