drm/i915/selftests: Fix build after dma-fence locking rework

drm/i915/selftests: Fix build after dma-fence locking rework

The i915_active selftest no longer builds after the dma-fence locking
rework because it directly accessed the fence’s spinlock. The helper
dma_fence_spinlock() must now be used to obtain the spinlock. Update the
selftest to use dma_fence_spinlock() accordingly.

Fixes: 1f32f310a13c ("dma-buf: inline spinlock for fence protection v5")
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patch.msgid.link/20260223172553.1663749-1-matthew.brost@intel.com