Replace xe_bo_populate()

We're publishing an xe_bo_populate() interface driver-wide, that calls into ttm_bo_populate(). Populating a ttm_bo is something that should be left to TTM and its backends, except if the driver needs TTM_PL_SYSTEM populated. Since we, unlike i915, don't bind to TTM_PL_SYSTEM but rather to TTM_PL_TT, we shouldn't be needing to populate directly.

It looks like code calling into xe_bo_populate() should instead use xe_bo_validate() to be consistent with other TTM drivers. Have a look at moving the existing code over to that interface and remove xe_bo_populate.