drm/xe: Use vfunc for pte/pde ppgtt encoding

Move the function to encode pte/pde to be vfuncs inside struct xe_vm.
This will allow to easily extend to platforms that don't have a
compatible encoding.

v2: Fix kunit build

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230927193902.2849159-4-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>