drm/xe/hw_engine_group: Add helper to wait for dma fence jobs

drm/xe/hw_engine_group: Add helper to wait for dma fence jobs

This is a required feature for faulting long running jobs not to be
submitted while dma fence jobs are running on the hw engine group.

v2: Switch to lockdep_assert_held_write in worker, get a proper reference
    for the last fence (Matt Brost)

v3: Directly call dma_fence_put with the fence ref (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/20240809155156.1955925-8-francois.dugast@intel.com