drm/amdgpu/gfx9: add gfxoff quirk

Fix screen corruption with firefox.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=207171


Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org