drm/amdgpu: pass NULL pointer instead of 0

Fixes: c030f2e4 ("drm/amdgpu: add amdgpu_ras.c to support ras (v2)")
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>