drm/amdgpu: retry dcpd fetch

Retry the dpcd fetch several times.  Some eDP panels
fail several times before the fetch is successful.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=73530

Ported from radeon.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/atombios_dp.c

@@ -358,11 +358,12 @@ int amdgpu_atombios_dp_get_dpcd(struct amdgpu_connector *amdgpu_connector)
 {
 	struct amdgpu_connector_atom_dig *dig_connector = amdgpu_connector->con_priv;
 	u8 msg[DP_DPCD_SIZE];
-	int ret;
+	int ret, i;
 
+	for (i = 0; i < 7; i++) {
 		ret = drm_dp_dpcd_read(&amdgpu_connector->ddc_bus->aux, DP_DPCD_REV, msg,
 				       DP_DPCD_SIZE);
-	if (ret > 0) {
+		if (ret == DP_DPCD_SIZE) {
 			memcpy(dig_connector->dpcd, msg, DP_DPCD_SIZE);
 
 			DRM_DEBUG_KMS("DPCD: %*ph\n", (int)sizeof(dig_connector->dpcd),
@@ -372,6 +373,7 @@ int amdgpu_atombios_dp_get_dpcd(struct amdgpu_connector *amdgpu_connector)
 
 			return 0;
 		}
+	}
 	dig_connector->dpcd[0] = 0;
 	return -EINVAL;
 }