Revert "amdgpu: don't call add_handle_to_table for KMS BO exports"

This reverts commit fe0488aa.

It caused messages like

 amdgpu 0000:23:00.0: bo 000000007dce0b3e va 0x0000101800-0x000010181f conflict with 0x0000101800-0x0000101820

in dmesg, and eventually a Xorg crash while running piglit.

Evidently, such BOs can actually be re-imported by other means than via
a KMS handle.