st/va: release held locks in error paths

Found with the help of following Coccinelle semantic patch:
// <smpl>
@@
expression E;
@@

  \(pthread_mutex_lock\|mtx_lock\|simple_mtx_lock\)(E)
  ...
(
  \(pthread_mutex_unlock\|mtx_unlock\|simple_mtx_unlock\)(E);
  ...
  return ...;
|
+ maybe need_unlock(E);
  return ...;
)
// </smpl>

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 0ad73031)