drm/i915: move FBC code out of i915_gem_stolen.c

With the abstractions created by the last patch, we can move this code
and the only thing inside intel_fbc.c that knows about dev_priv->mm is
the code that reads stolen_base.

We also had to move a call to i915_gem_stolen_cleanup_compression()
- now called intel_fbc_cleanup_cfb() - outside i915_gem_stolen.c.

v2:
  - Rebase after the remove_node() changes on the previous patch.

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>