Various TTM cleanups/fixes

This series includes mostly no-functional-changes to simplify
or cleanup various routines.

Patch #11 includes an fix to functional behaviour.

Follow-Ups:
- Various TTM cleanups/fixes
  - From: Christian König
- [PATCH 12/12] drm/ttm: Simplify ttm_eu_reserve_buffers()
  - From: Tom St Denis
- [PATCH 11/12] drm/ttm: Fix 'buf' pointer update in ttm_bo_vm_access_kmap()
  - From: Tom St Denis
- [PATCH 10/12] drm/ttm: Remove unncessary retval from ttm_bo_vm_fault()
  - From: Tom St Denis
- [PATCH 09/12] drm/ttm: Fix indentation in ttm_bo_move_memcpy()
  - From: Tom St Denis
- [PATCH 08/12] drm/ttm: Fix indentation in ttm_dma_pool_alloc_new_pages()
  - From: Tom St Denis
- [PATCH 07/12] drm/ttm: Simplify ttm_dma_find_pool()
  - From: Tom St Denis
- [PATCH 06/12] drm/ttm: Simplify ttm_dma_page_put()
  - From: Tom St Denis
- [PATCH 05/12] drm/ttm: Fix indentation in ttm_pool_store()
  - From: Tom St Denis
- [PATCH 04/12] drm/ttm: Fix indentation in ttm_tt_swapout()
  - From: Tom St Denis
- [PATCH 03/12] drm/ttm: Change ttm_tt page allocations to return errors
  - From: Tom St Denis
- [PATCH 02/12] drm/ttm: Add a default BO destructor to simplify code
  - From: Tom St Denis
- [PATCH 01/12] drm/ttm: Clean up indentation in ttm_bo.c
  - From: Tom St Denis