drm/amdkfd: call amdgpu_amdkfd_get_unique_id directly

No need to use a function pointer because the implementation is not
ASIC-specific. This fixes missing support due to a missing function
pointer on Arcturus.

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