drm/amdgpu: make the WREG32_SOC15_xx macro to support multi GC

To write regs on different GCDs, use the inst index.

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Le Ma <Le.Ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>