drm/i915: Make shrink/unshrink be atomic

Add an atomic counter and always take the spinlock around the pin/unpin
events, so that we can perform the list manipulation concurrently.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910212204.17190-1-chris@chris-wilson.co.uk