drm/i915: Fix error pointer dereference in i915_gem_do_execbuffer()

Originally "out_fence" was set using out_fence = sync_file_create() but
which returns NULL, but now it is set with out_fence = eb_requests_create()
which returns error pointers.  The error path needs to be modified to
avoid an Oops in the "goto err_request;" path.

Fixes: 544460c3 ("drm/i915: Multi-BB execbuf")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211202044831.29583-1-matthew.brost@intel.com

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -3366,6 +3366,7 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 	out_fence = eb_requests_create(&eb, in_fence, out_fence_fd);
 	if (IS_ERR(out_fence)) {
 		err = PTR_ERR(out_fence);
+		out_fence = NULL;
 		if (eb.requests[0])
 			goto err_request;
 		else