drm/amdgpu: Fix register access on GC v9.4.3

In GC v9.4.3 there are multiple XCCs. It's required to use
physical instance number to get the right register offset. Use
GET_INST API for that.

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>