The locking rework/fix that I promised when I commited GEM.

Before, as well as being kinda nasty there was a very definite race, if
the last reference to an object  was removed by uvm (a map going away),
then the free path happened unlocked, this could cause all kinds of
havoc.

In order to deal with this, move to fine-grained locking. Since uvm
object locks are spinlocks, and we need to sleep in operations that will
wait on the gpu, provide a DRM_BUSY flag that is set on a locked object
that then allows us to unlock and sleep (this is similar to several
things done in uvm on pages and some object types).

The rwlock stays around to ensure that execbuffer can have acces to the
whole gtt, so ioctls that bind to the gtt need a read lock, and
execuffer gets a write lock. otherwise most ioctls just need to busy the
object that they operate on. Lists also have their own locks.

Some cleanup could be done to make this a little prettier, but it is
much more correct than previously.

Tested very very vigorously on 855 (x40) and 965 (x61s), this found numerous
bugs. Also, the I can no longer crash the kernel at will.

A bunch of asserts hidden under DRMLOCKDEBUG have been left in the code for
debugging purposes.

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