drm/amdgpu: Fix mode2 reset for sienna cichlid

Before this change, sienna_cichlid_get_reset_handler will always
return NULL, although the module parameter reset_method is 3
when loading amdgpu driver.

Signed-off-by: lyndonli <Lyndon.Li@amd.com>
Signed-off-by: Yunxiang Li <Yunxiang.Li@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>