drm/amd/display: Convert connector signal id to string

To improve the readability of the of the log, this commit introduces a
function that converts the signal type id to a human-readable string.

Reviewed-by: Jerry Zuo <jerry.zuo@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>