drm: implement a method to free unused pages

drm: implement a method to free unused pages

On contiguous allocation, we round up the size
to the *next* power of 2, implement a function
to free the unused pages after the newly allocate block.

v2(Matthew Auld):
  - replace function name 'drm_buddy_free_unused_pages' with
    drm_buddy_block_trim
  - replace input argument name 'actual_size' with 'new_size'
  - add more validation checks for input arguments
  - add overlaps check to avoid needless searching and splitting
  - merged the below patch to see the feature in action
     - add free unused pages support to i915 driver
  - lock drm_buddy_block_trim() function as it calls mark_free/mark_split
    are all globally visible

v3(Matthew Auld):
  - remove trim method error handling as we address the failure case
    at drm_buddy_block_trim() function

v4:
  - in case of trim, at __alloc_range() split_block failure path
    marks the block as free and removes it from the original list,
    potentially also freeing it, to overcome this problem, we turn
    the drm_buddy_block_trim() input node into a temporary node to
    prevent recursively freeing itself, but still retain the
    un-splitting/freeing of the other nodes(Matthew Auld)

  - modify the drm_buddy_block_trim() function return type

v5(Matthew Auld):
  - revert drm_buddy_block_trim() function return type changes in v4
  - modify drm_buddy_block_trim() passing argument n_pages to original_size
    as n_pages has already been rounded up to the next power-of-two and
    passing n_pages results noop

v6:
  - fix warnings reported by kernel test robot <lkp@intel.com>

v7:
  - modify drm_buddy_block_trim() function doc description
  - at drm_buddy_block_trim() handle non-allocated block as
    a serious programmer error
  - fix a typo

Signed-off-by: Arunpravin <Arunpravin.PaneerSelvam@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221164552.2434-3-Arunpravin.PaneerSelvam@amd.com
Signed-off-by: Christian König <christian.koenig@amd.com>