drm/amdgpu: only enable swiotlb alloc when need v2

get the max io mapping address of system memory to see if it is over
our card accessing range.
v2: move checking 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/20180209024410.1469-2-david1.zhou@amd.com