drm/xe: Simplify xe_res_get_buddy()

We can remove the unnecessary indirection thru xe->tiles[] to get
the TTM VRAM manager.  This code can be common for VRAM and STOLEN.

Signed-off-by: Brian Welty <brian.welty@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>