drm/i915/ttm: add ttm_buddy_man

drm/i915/ttm: add ttm_buddy_man

Add back our standalone i915_buddy allocator and integrate it into a
ttm_resource_manager. This will plug into our ttm backend for managing
device local-memory in the next couple of patches.

v2(Thomas):
    - Return -ENOSPC from the buddy; ttm expects this in order to
      trigger eviction
    - Drop the unnecessary inline
    - bo->page_alignment is in page units

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616152501.394518-1-matthew.auld@intel.com