drm/xe: Add ULLS migration job support to ring ops

Add preamble and postamble for ULLS migrations jobs. Preamble clears
current semaphore for reuse. Postamble waits on next semaphore which is
set upon next job submission, then advances the ring tail over that job
with an LRI to RING_TAIL, so submitting it costs the CPU nothing beyond
signalling the semaphore.

A job updates the tail on behalf of a successor which has not been
emitted yet, so it cannot know how much ring that successor will occupy.
Pad every ULLS job out to a fixed ULLS_JOB_SIZE_BYTES, which makes the
next tail derivable from where the current job starts. The pad also
supplies the NOPs which must follow an in-ring tail update.

The last ULLS migration job skips BB submission, the postamble and the
tail update (clear current semaphore, write seqno, exit ULLS), padding
the difference so that it still fills a job slot.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Maarten Lankhorst <dev@lankhorst.se>
Link: https://patch.msgid.link/20260925201526.1355279-21-matthew.brost@intel.com

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