drm/xe/bo: optimise TT population for DONTNEED BOs

When a VRAM buffer object is marked as DONTNEED, like in Mesa, the
driver skips migrating its contents to system memory and instead purges
the backing store during eviction (via xe_ttm_bo_purge).

However, xe_evict_flags() still returns tt_placement (XE_PL_TT) for VRAM
BOs even if they were marked DONTNEED.  This causes
ttm_bo_handle_move_mem() to always call ttm_bo_populate() to allocate
destination system pages, only for those pages to be immediately freed
right after when xe_bo_move() calls xe_ttm_bo_purge().

Fix this by changing xe_evict_flags() to return sys_placement
(XE_PL_SYSTEM) for DONTNEED BOs.  This causes TTM to skip the population
step while still ensuring that TTM calls xe_bo_move(), so
xe_ttm_bo_purge() still triggers.

v2 (Thomas):
  - Add a comment to highlight why sys_placement over purge_placement.

Assisted-by: Copilot:gemini-3.1-pro-preview
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Maarten Lankhorst <dev@lankhorst.se>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260723161542.1220276-2-matthew.auld@intel.com

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