drm/amdgpu: disable gfxoff for navy_flounder

gfxoff is temporarily disabled for navy_flounder,
since at present the feature has broken some basic
amdgpu test.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>