drm/amdgpu: Initialize umc ras callback

Fix a coding error which results to null interrupt
handler for umc ras.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Stanley Yang <Stanley.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>