drm/amdgpu: disable gfxoff on navi10

The gfxoff brings unstability, disable it by default

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: tiancyin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>