drm/amdgpu: Simplify the allocation of sync slab caches

Use the new KMEM_CACHE() macro instead of direct kmem_cache_create
to simplify the creation of SLAB caches.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>