drm/amdgpu: fix amdgpu_coredump

The VM pointer might already be outdated when that function is called.
Use the PASID instead to gather the information instead.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>