drm/amdgpu: Use status register for partition mode

Program partition status register to reflect the current partition mode.
Partition capability register is for capability and is a one-time setting.

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