drm/i915: i915_gem_object_finish_gtt must always release gtt mmap

Even if the object is no longer in the GTT domain, there may still be
a user space mapping which needs to be released.

Without this fix, render-based text (mostly in firefox) would
occasionally get corrupted when the system was under load.

Signed-off-by: Keith Packard <keithp@keithp.com>