drm/ttm: move the page_alignment into the BO v2

The alignment is a constant property and shouldn't change.

v2: move documentation as well as suggested by Matthew.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210413135248.1266-4-christian.koenig@amd.com