drm/ttm: Use private locks for the default bo range manager

Searching for a free block in the range manager may in some situations be a
lenghty operation, and we want to avoid holding the global lru lock
during that time. Instead use a per-manager spinlock.

This leaves the global lru lock for quick lru list and swap list manipulation
only, including list manipulation associated with reserving buffer objects.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>