drm/i915: add simple wrappers for stolen node insertion/removal

We want to move the FBC code out of i915_gem_stolen.c, but that code
directly adds/removes stolen memory nodes. Let's create this
abstraction, so i915_gme_stolen.c is still in control of all the
stolen memory handling. The abstraction will also allow us to add
locking assertions later.

v2:
  - Add dev_priv as remove_node() argument since we'll need it later
    (Chris).

Requested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>