drm/amdgpu: svm bo enable_signal call condition

drm/amdgpu: svm bo enable_signal call condition

[why]
To support svm bo eviction mechanism.

[how]
If the BO crated has AMDGPU_AMDKFD_CREATE_SVM_BO flag set,
enable_signal callback will be called inside amdgpu_evict_flags.
This also causes gutting of the BO by removing all placements,
so that TTM won't actually do an eviction. Instead it will discard
the memory held by the BO. This is needed for HMM migration to user
mode system memory pages.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>