drm/dp_mst: Fix return code on sideband message failure

Commit 2f015ec6 ("drm/dp_mst: Add sideband down request tracing +
selftests") added some debug code for sideband message tracing. But
it seems to have unintentionally changed the behavior on sideband message
failure. It catches and returns failure only if DRM_UT_DP is enabled.
Otherwise it ignores the error code and returns success. So on an MST
unplug, the caller is unaware that the clear payload message failed and
ends up waiting for 4 seconds for the response. Fixes the issue by
returning the proper error code.

Changes in V2:
-- Revise commit text as review comment
-- add Fixes text

Changes in V3:
-- remove "unlikely" optimization

Fixes: 2f015ec6 ("drm/dp_mst: Add sideband down request tracing + selftests")
Cc: <stable@vger.kernel.org> # v5.5+
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1625585434-9562-1-git-send-email-khsieh@codeaurora.org

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -2872,11 +2872,13 @@ static int process_single_tx_qlock(struct drm_dp_mst_topology_mgr *mgr,
 	idx += tosend + 1;
 
 	ret = drm_dp_send_sideband_msg(mgr, up, chunk, idx);
-	if (unlikely(ret) && drm_debug_enabled(DRM_UT_DP)) {
-		struct drm_printer p = drm_debug_printer(DBG_PREFIX);
+	if (ret) {
+		if (drm_debug_enabled(DRM_UT_DP)) {
+			struct drm_printer p = drm_debug_printer(DBG_PREFIX);
 
-		drm_printf(&p, "sideband msg failed to send\n");
-		drm_dp_mst_dump_sideband_msg_tx(&p, txmsg);
+			drm_printf(&p, "sideband msg failed to send\n");
+			drm_dp_mst_dump_sideband_msg_tx(&p, txmsg);
+		}
 		return ret;
 	}