drm/xe/vm: split userptr bits into separate file

drm/xe/vm: split userptr bits into separate file

This will simplify compiling out the bits that depend on DRM_GPUSVM in a
later patch. Without this we end up littering the code with ifdef
checks, plus it becomes hard to be sure that something won't blow at
runtime due to something not being initialised, even though it passed
the build. Should be no functional change here.

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: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250828142430.615826-16-matthew.auld@intel.com