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

[ Upstream commit a887adad ]

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
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

drivers/gpu/drm/radeon/radeon_connectors.c

@@ -72,6 +72,11 @@ void radeon_connector_hotplug(struct drm_connector *connector)
 			if (!radeon_hpd_sense(rdev, radeon_connector->hpd.hpd)) {
 				drm_helper_connector_dpms(connector, DRM_MODE_DPMS_OFF);
 			} else if (radeon_dp_needs_link_train(radeon_connector)) {
+				/* Don't try to start link training before we
+				 * have the dpcd */
+				if (!radeon_dp_getdpcd(radeon_connector))
+					return;
+
 				/* set it to OFF so that drm_helper_connector_dpms()
 				 * won't return immediately since the current state
 				 * is ON at this point.