drm/amdgpu: disable gfxoff when using register read interface

When gfxoff is enabled, accessing gfx registers via MMIO
can lead to a hang.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=205497


Acked-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org