drm/amdgpu: Fix the null pointer dereference to ras_manager

[ Upstream commit 4c11d30c95576937c6c35e6f29884761f2dddb43 ]

Check ras_manager before using it

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>