drm/amdgpu: remove amdgpu_cs_try_evict

Trying to evict things from the current working set doesn't work that
well anymore because of per VM BOs.

Rely on reserving VRAM for page tables to avoid contention.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>