drm/ttm: revert "Reduce the number of used allocation orders for TTM pages"

This reverts commit 322458c2.

PMD_SHIFT is not necessary constant on all architectures resulting in
build failures.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/CAKMK7uHgUuqWJuqmZKrxi2mNiqExhmMif-naYnzUSj-puW-x+A@mail.gmail.com