drm/xe: Remove check for vma == NULL

vma at this point can never be NULL as otherwise it would crash earlier
in the only caller, xe_pt_stage_bind_entry(). Remove the extra check and
avoid adding and removing the bits from the pte.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230927193902.2849159-3-lucas.demarchi@intel.com


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