drm/amd/display: Add GAMCOR memory shutdown support

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

[How]
Write to GAMCOR_MEM_PWR_FORCE to put memory to shutdown
when GAMCOR is not used.

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>