drm/amdgpu: partially revert "svm bo enable_signal call condition"

drm/amdgpu: partially revert "svm bo enable_signal call condition"

Partially revert commit a5837065aedcbc6f405fbe6dc6ebb55852ef4444.

First of all this is illegal use of RCU to call dma_fence_enable_sw_signaling()
since we don't hold a reference to the fence in question and can crash badly.

Then the code doesn't seem to have the intended effect since only the
exclusive fence is handled, but the KFD fences are always added as shared fence.

Only keep the handling to throw away the content of SVM BOs.

Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211122123926.385017-1-christian.koenig@amd.com
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>