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
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu.h b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
index d5a2eef..31126df 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu.h
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
@@ -1504,6 +1504,7 @@ struct amdgpu_device {
 	const struct amdgpu_asic_funcs	*asic_funcs;
 	bool				shutdown;
 	bool				need_dma32;
+	bool				need_swiotlb;
 	bool				accel_working;
 	struct work_struct		reset_work;
 	struct notifier_block		acpi_nb;