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

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: 544460c33821 ("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