drm/i915: Only unwind the local pgtable layer if empty

Only if we allocated the layer and the lower level failed should we
remove this layer when unwinding. Otherwise we ignore the overlapping
entries by overwriting the old layer with scratch.

Fixes: c5d092a4 ("drm/i915: Remove bitmap tracking for used-pml4")
Fixes: e2b763ca ("drm/i915: Remove bitmap tracking for used-pdpes")
Reported-by: Matthew Auld <matthew.william.auld@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99947


Testcase: igt/drv_selftest/live_gtt
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Tested-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227122654.27651-1-chris@chris-wilson.co.uk