drm/amdgpu: Resolve pcie_bif RAS recovery bug

Check shared buf instead of init flag for xgmi ta shared buf init
during xgmi ta initialization.

Signed-off-by: Candice Li <candice.li@amd.com>
Reviewed-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>