drm/amd/display: Add registry for mem pwr control

[What]
Need debug options to control lightl/deep sleep
[How]
Add registry for memory power control

Signed-off-by: Braden Bakker <Braden.Bakker@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>