drm/xe: Fix unbind of unaccessed VMA (fault mode)

In fault mode, page table binding is deferred until fault handler.
Thus vma->tile_present will be unset unless the VMA is accessed by GPU.

During a later unbind, the logic doesn't account for the fact that local
fence variable will be NULL in this case, leading to pass NULL into
dma_fence_add_callback() and causing few WARN_ONs to print to console.
The fix is already present in the code, just hoist the fence variable
computation to be done earlier.

Resolves warnings seen with igt@xe_exec_fault_mode@once-invalid-fault

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>