drm/xe: Split xe_migrate allocation from initialization

drm/xe: Split xe_migrate allocation from initialization

Currently, xe_migrate_init handled both allocation and initialization,
Lets moves allocation to xe_tile_alloc via a new xe_migrate_alloc
function, and keep initialization in xe_migrate_init.
This will allow the migration pointers to be passed to other structures
before full initialization.
Also replaces devm_kzalloc with drmm_kzalloc for better
DRM-managed memory.

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250714184818.89201-5-piotr.piorkowski@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>