drm/amdgpu: Attach eviction fence on alloc

Instead of attaching the eviction fence when a KFD BO is first mapped,
attach it when it is allocated or imported. This in preparation to allow
KFD BOs to be mapped using the render node API.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>