drm/xe: Fix NULL pointer dereference in xe_exec_ioctl

drm/xe: Fix NULL pointer dereference in xe_exec_ioctl

Helper function xe_sync_needs_wait expects sync->fence when accessing
flags, patch makes sure we call only when sync->fence exists.

v2: move null checking to xe_sync_needs_wait and make
    xe_sync_entry_wait utilize this helper (Matthew Auld)
v3: further simplify code (Matthew Auld)

Fixes NULL pointer dereference seen with Vulkan workloads:

[  118.410401] RIP: 0010:xe_sync_needs_wait+0x27/0x50 [xe]

Fixes: 4ac9048d0501 ("drm/xe: Wait on in-syncs when swicthing to dma-fence mode")
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251217132412.435755-1-tapani.palli@intel.com