drm/amdgpu: add new HDP CG flags

HDP 5.0 supports SRAM power gating. all the LS (Light Sleep)/
DS (Deep Sleep)/SD (Shut Down) modes are supported. However,
only one of these modes can be enabled at one time.

There is no dynamic power mode switch support. clock/power gating
has to be disabled before making any power mode change.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>