drm/exec: don't immediately add prelocked obj to array of locked objs

In the Linux kernel, the following vulnerability has been resolved:

drm/exec: don't immediately add prelocked obj to array of locked objs

When a contention is detected the drm_exec object grabs a reference to
the contended object, unlocks all other objects and pre-locks the
contended one before anything else.

The problem is that it is possible that this pre-locked object was about
to be destroyed and drm_exec is now holding the last reference to it.

So we never actually see this pre-locked object again to be explicitly
locked and so also never reserve a slot for a dma_fence object on it.

Should the driver now use drm_exec_for_each_locked_object() to add a new
fence to all locked objects we eventually see a warning or even a
BUG_ON() to prevent random memory corruption.

Fix this by keeping the prelocked object out of the array of locked objs
until it was explicitly mentioned by the driver again.

If the prelocked object is never mentioned again by the driver
just unlock and drop the reference at the end of the locking loop.