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

[Why]
The OPTC memory blocks should be powered down when they are not in use.
This will reduce power consumption.

[How]
1. Set ODM_MEM_UNASSIGNED_PWR_MODE to shutdown memory when unassigned
2. Set ODM_MEM_VBLANK_PWR_MODE to light sleep mode when in vblank
3. Added a debug option to allow this behaviour to be turned off
4. Restructured debug options to use a bitfield in a way that's more clear

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