drm/syncobj: fix leaking dma_fence in drm_syncobj_query_ioctl

We need to check the context number instead if the previous sequence to detect
an error and if an error is detected we need to drop the reference to the
current fence or otherwise would leak it.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 27b575a9 ("drm/syncobj: add timeline payload query ioctl v6")
Link: https://patchwork.freedesktop.org/patch/319123/