drm/amdgpu:Preamble is forbid to be ignored in SRIOV

SR-IOV requires the preamble.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>