drm/sched: Increment job count before swapping tail spsc queue

A small race exists between spsc_queue_push and the run-job worker, in
which spsc_queue_push may return not-first while the run-job worker has
already idled due to the job count being zero. If this race occurs, job
scheduling stops, leading to hangs while waiting on the job’s DMA
fences.

Seal this race by incrementing the job count before appending to the
SPSC queue.

This race was observed on a drm-tip 6.16-rc1 build with the Xe driver in
an SVM test case.

Fixes: 1b1f42d8 ("drm: move amd_gpu_scheduler into common location")
Fixes: 27105db6 ("drm/amdgpu: Add SPSC queue to scheduler.")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://lore.kernel.org/r/20250613212013.719312-1-matthew.brost@intel.com