drm/msm: Drop struct_mutex from the retire path

Now that we are not relying on dev->struct_mutex to protect the
ring->submits lists, drop the struct_mutex lock.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>