drm/ttm: Tidy usage of local variables a little bit

drm/ttm: Tidy usage of local variables a little bit

At the moment the TTM code has a few places which exibit sub-optimal
patterns regarding local variable usage:

 * Having a local with some object cached but not always using it.
 * Having a local for a single use object member access.
 * Failed opportunities to use a local to cache a pointer.

Lets tidy this a little bit and apply some more consistency.

It is mostly for consistency and redability but I have also checked that
there are not negative code generation effects. In fact there are more
positives:

add/remove: 0/0 grow/shrink: 3/9 up/down: 12/-175 (-163)
Function                                     old     new   delta
ttm_pool_restore_and_alloc                   415     423      +8
ttm_bo_vunmap                                147     149      +2
ttm_bo_evict                                 521     523      +2
ttm_bo_vm_fault_reserved                     972     970      -2
ttm_bo_vm_dummy_page                         155     152      -3
ttm_bo_vm_fault                              203     196      -7
ttm_bo_populate                              158     150      -8
ttm_bo_move_memcpy                           600     592      -8
ttm_bo_kmap                                  667     644     -23
ttm_bo_shrink                                333     305     -28
ttm_bo_release                               750     720     -30
ttm_bo_swapout_cb                            691     625     -66
Total: Before=42717, After=42554, chg -0.38%

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Link: https://lore.kernel.org/r/20250919131530.91247-5-tvrtko.ursulin@igalia.com
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
[tursulin: fixup conflict in ttm_bo_move_pipeline_evict]