drm/amdgpu: Fix programming of initial XCP mode

On initialization set the partition mode correctly to SPX (default) or
any other user specified partition mode. Use switch_compute_partition
API so that all settings are initialized correctly.

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>