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

Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240912153842.35813-1-jose.souza@intel.com