drm/xe/exec_queue: Prepare last fence for hw engine group resume context

Ensure we can safely take a ref of the exec queue's last fence from the
context of resuming jobs from the hw engine group. The locking requirements
differ from the general case, hence the introduction of this new function.

v2: Add kernel doc, rework the code to prevent code duplication

v3: Fix kernel doc, remove now unnecessary lockdep variants (Matt Brost)

v4: Remove new put function (Matt Brost)

Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240809155156.1955925-7-francois.dugast@intel.com