drm/radeon: only enable swiotlb path when need v2

swiotlb expands our card accessing range, but its path always is slower
than ttm pool allocation.
So add condition to use it.
v2: move a bit later

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180209024410.1469-3-david1.zhou@amd.com