drm/xe/oa: Separate batch submission from waiting for completion

When we introduce xe_syncs, we don't wait for internal OA programming
batches to complete. That is, xe_syncs are signaled asynchronously. In
anticipation for this, separate out batch submission from waiting for
completion of those batches.

v2: Change return type of xe_oa_submit_bb to "struct dma_fence *" (Matt B)
v3: Retain init "int err = 0;" in xe_oa_submit_bb (Jose)

Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241022200352.1192560-2-ashutosh.dixit@intel.com