drm/i915/selftests: Do not overwrite error code after intel_context_migrate_clear() call

Currently this function's error code is stored in err variable,
which, if a i915_request is present, will be immediately
overwritten by return from dma_resv_reserve_fences(). Call DMA
functions only if intel_context_migrate_clear() succeeded.

Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/c0348e052bcb2870acdc15d60f5389fbdf4ef886.1753859971.git.krzysztof.karas@intel.com

Author: KKaras169169

drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c

@@ -1228,7 +1228,7 @@ static int __igt_mmap_migrate(struct intel_memory_region **placements,
 					  i915_gem_object_is_lmem(obj),
 					  expand32(POISON_INUSE), &rq);
 	i915_gem_object_unpin_pages(obj);
-	if (rq) {
+	if (rq && !err) {
 		err = dma_resv_reserve_fences(obj->base.resv, 1);
 		if (!err)
 			dma_resv_add_fence(obj->base.resv, &rq->fence,