drm/amd/powerplay: cache the watermark settings on system memory

So that we do not need to allocate a piece of VRAM for it. This
is a preparation for coming change which unifies the VRAM address
for all driver tables interaction with SMU.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>