drm/xe/oa: Move default oa unit assignment earlier during stream open

drm/xe/oa: Move default oa unit assignment earlier during stream open

De-referencing param.oa_unit, when an OA unit id is not provided during
stream open, results in NPD below.

  Oops: general protection fault, probably for non-canonical address...
  KASAN: null-ptr-deref in range...
  RIP: 0010:xe_oa_stream_open_ioctl+0x169/0x38a0
   xe_observation_ioctl+0x19f/0x270
   drm_ioctl_kernel+0x1f4/0x410

Fix this by moving default oa unit assignment before the dereference.

Reported-by: Peter Senna Tschudin <peter.senna@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6840
Fixes: c7e269aa565f ("drm/xe/oa: Allow exec_queue's to be specified only for OAG OA unit")
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Harish Chegondi <harish.chegondi@intel.com>
Link: https://patch.msgid.link/20251212061850.1565459-2-ashutosh.dixit@intel.com