drm/i915: Don't try to tear down the stolen drm_mm if it's not there

Every other place properly checks whether we've managed to set
up the stolen allocator at boot-up properly, with the exception
of the cleanup code. Which results in an ugly

*ERROR* Memory manager not clean. Delaying takedown

at module unload time since the drm_mm isn't initialized at all.

v2: While at it check whether the stolen drm_mm is initialized instead
of the more obscure stolen_base == 0 check.

v3: Fix up the logic. Also we need to keep the stolen_base check in
i915_gem_object_create_stolen_for_preallocated since that can be
called before stolen memory is fully set up. Spotted by Chris Wilson.

v4: Readd the conversion in i915_gem_object_create_stolen_for_preallocated,
the check is for the dev_priv->mm.gtt_space drm_mm, the stolen
allocatot must already be initialized when calling that function (if
we indeed have stolen memory).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65953


Cc: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: lu hua <huax.lu@intel.com> (v3)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>