drm: fix mutex leak in drm_dp_get_mst_branch_device

In Linux 4.3-rc5, there is an error case in drm_dp_get_branch_device
that returns without releasing mgr->lock, resulting a spew of kernel
messages about a kernel work function possibly having leaked a mutex
and presumably more serious adverse consequences later.  This patch
changes the error to "goto out" to unlock the mutex before returning.

Signed-off-by: Adam J. Richter <adam_richter2004@yahoo.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>