drm/amdgpu: use mode1 reset by default for sienna_cichlid

Swith default gpu reset method for sienna_cichlid to MODE1 reset.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>