drm/amdgpu: disable gfxoff on original raven

There are still combinations of sbios and firmware that
are not stable.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=204689


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