drm/amd/display: Add DMCU memory low power support

[Why]
On some platforms, DMCU is no longer used. In these cases, some DMCU
memory should be completely powered off to save power.

[How]
1. Set DMCU_ERAM_MEM_PWR_FORCE to shutdown memory when DMCU is not in
   use
2. Added a debug option to allow this behaviour to be turned off
3. Set all memory low power debug options to off first, to not
   immediately cause problems

Signed-off-by: Jacky Liao <ziyu.liao@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>