drm/i915/ttm: restore min_page_size behaviour

We now have bo->page_alignment which perfectly describes what we need if
we have min page size restrictions for lmem. We can also drop the flag
here, since this is the default behaviour for all objects.

v2(Thomas):
    - bo->page_alignment is in page units

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616152501.394518-7-matthew.auld@intel.com