drm/amdkfd: add svm_bo eviction mechanism support

svm_bo eviction mechanism is different from regular BOs.
Every SVM_BO created contains one eviction fence and one
worker item for eviction process.
SVM_BOs can be attached to one or more pranges.
For SVM_BO eviction mechanism, TTM will start to call
enable_signal callback for every SVM_BO until VRAM space
is available.
Here, all the ttm_evict calls are synchronous, this guarantees
that each eviction has completed and the fence has signaled before
it returns.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Signed-off-by: Philip Yang <Philip.Yang@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>