drm/i915: pre-alloc instead of drm_mm search/get_block

i915 is the last user of the weird search+get_block drm_mm API. Convert it
to an explicit kmalloc()+insert_node(). This drops the last user of the
node-cache in drm_mm. We can remove it now in a follow-up patch.

v2:
 - simplify error path in i915_setup_compression()
v3:
 - simplify error path even more

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>