drm/amdgpu: check atomic flag to differeniate with legacy path

since vkms support atomic KMS interface

Signed-off-by: Flora Cui <flora.cui@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Acked-by: Alex Deucher <aleander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>