drm/amdgpu/gfx10: Disable gfxoff before disabling powergating.

Otherwise we get a full system lock (looks like a FW mess).

Copied the order from the GFX9 powergating code.

Fixes: 366468ff ("drm/amdgpu: Allow GfxOff on Vangogh as default")
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2545


Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org