drm/xe/oa: Fix overflow in oa batch buffer

In the Linux kernel, the following vulnerability has been resolved:

drm/xe/oa: Fix overflow in oa batch buffer

By default xe_bb_create_job() appends a MI_BATCH_BUFFER_END to batch buffer, this is not a problem if batch buffer is only used once but oa reuses the batch buffer for the same metric and at each call it appends a MI_BATCH_BUFFER_END, printing the warning below and then overflowing.

[ 381.072016] ------------[ cut here ]------------ [ 381.072019] xe 0000:00:02.0: [drm] Assertion bb->len * 4 + bb_prefetch(q->gt) <= size failed! platform: LUNARLAKE subplatform: 1 graphics: Xe2_LPG / Xe2_HPG 20.04 step B0 media: Xe2_LPM / Xe2_HPM 20.00 step B0 tile: 0 VRAM 0 B GT: 0 type 1

So here checking if batch buffer already have MI_BATCH_BUFFER_END if not append it.

v2: - simply fix, suggestion from Ashutosh

(cherry picked from commit 9ba0e0f30ca42a98af3689460063edfb6315718a)