drm: use after free in drm_queue_vblank_event()

The "e" pointer is either NULL or freed when we call
drm_vblank_put(dev, e->pipe) on the error path.  Just pass the "pipe"
variable directly instead.

I changed another caller to use "pipe" as well for consistency.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>