drm/amd/display: Add BLNDGAM memory shutdown support

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

[How]
1. Write to BLNDGAM_MEM_PWR_FORCE to put memory to shutdown when BLNDGAM
   is not used.
2. Added a debug option to allow this behaviour to be turned off

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>