drm/msm: split locking and pinning BO's

Split up locking and pinning buffers in the submit path.  This is needed
because we'll want to insert fencing in between the two steps.

This makes things end up looking more similar to etnaviv submit code
(which was originally modelled on the msm code but has already added
'struct fence' support).

Signed-off-by: Rob Clark <robdclark@gmail.com>