drm/xe: Do not preempt fence signaling CS instructions

drm/xe: Do not preempt fence signaling CS instructions

If a batch buffer is complete, it makes little sense to preempt the
fence signaling instructions in the ring, as the largest portion of the
work (the batch buffer) is already done and fence signaling consists of
only a few instructions. If these instructions are preempted, the GuC
would need to perform a context switch just to signal the fence, which
is costly and delays fence signaling. Avoid this scenario by disabling
preemption immediately after the BB start instruction and re-enabling it
after executing the fence signaling instructions.

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Carlos Santa <carlos.santa@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patch.msgid.link/20260115004546.58060-1-matthew.brost@intel.com