drm/amd/display: Adjust dto fix coverage

[WHY & HOW]
Unnecessary to affect legacy APU's dto src sel during dpms behaviour

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Ausef Yousof <Ausef.Yousof@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>