drm/amd/display: Select correct DTO source

[WHY&HOW]
Change criteria for setting DTO source value, and always set it regardless of
the signal type.

Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>