drm/amdgpu: remove TOPDOWN flags when allocating VRAM in large bar system

Since VRAM manager is changed from drm mm to drm buddy, the
TOP_DOWN flag should not be set by default in the large bar system.
Removing this flag helps improve drm buddy allocator efficiency and
reduce the risk of splitting higher order block into lower order.

Signed-off-by: Shane Xiao <shane.xiao@amd.com>
Reviewed-by: Christian K�nig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>