drm/xe: Misc refine for svm

These changes should have no functional impact.
1. Correct typo of "operation"in macro range_debug().
2. Combine 2 spin_lock() call in xe_svm_garbage_collector() into 1.
3. Drop redundant preferred_region_is_vram check in
   xe_svm_range_needs_migrate_to_vram().
4. Combine the devmem_possible check in xe_svm_handle_pagefault().
   need_vram includes the IS_DGFX() check, so there is no change for
   .devmem_only.

v2: revert !ctx.devmem_only change (Matt)
v3: rebase code and refine commit message.
v4: rebase code and refine commit message.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20250911031405.1371812-2-shuicheng.lin@intel.com