drm/amd/display: log underflow

as well as convert output in ref_cycle to nano sec

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Logatharshan Thothiralingam <Logatharshan.Thothiralingam@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>