drm/amd/display: fix missing reg offset

[Why]
Initializing was missing reg offsets for the dmcub test debug registers
causing assert

[How]
Add initialization

Reviewed-by: Kazlauskas Nicholas <Nicholas.Kazlauskas@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>