drm/msm: Refcount submits

Before we remove dev->struct_mutex from the retire path, we have to deal
with the situation of a submit retiring before the submit ioctl returns.

To deal with this, ring->submits will hold a reference to the submit,
which is dropped when the submit is retired.  And the submit ioctl path
holds it's own ref, which it drops when it is done with the submit.

Also, add to submit list *after* getting/pinning bo's, to prevent badness
in case the completed fence is corrupted, and retire_worker mistakenly
believes the submit is done too early.

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>