drm/amd/display: Add MST debug message when link detection fails

[WHY & HOW]
dc_link_detect returns a boolean value which can be used to print debug
messages when it fails.

This fixes 1 CHECKED_RETURN issue reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>