drm/xe/svm: Add xe_svm_find_vma_start() helper

Add helper xe_svm_find_vma_start() function to determine start of cpu
vma in input range.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://lore.kernel.org/r/20250513040228.470682-14-himal.prasad.ghimiray@intel.com


Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>