drm/ttm: stop using GFP_TRANSHUGE_LIGHT

drm/ttm: stop using GFP_TRANSHUGE_LIGHT

The only flag we really need is __GFP_NOMEMALLOC, highmem depends on
dma32 and moveable/compound should never be set in the first place.

Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/413812/
Link: https://patchwork.freedesktop.org/patch/413964/
Fixes: d099fc8f540a ("drm/ttm: new TT backend allocation pool v3")
Reported-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>