drm/ttm: don't use compound pages for now

We need to figure out first how to correctly map them into the CPU page tables.

bug: https://bugs.freedesktop.org/show_bug.cgi?id=103138


Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>