drm/amd/powerplay: disable gfxoff for navi14

gfxoff doesn't work on navi14 yet, so disable it for now

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>