drm/msm: small mmap offset cleanups

Use pre-computed iova when unmapping, to reduce the places we assume iova
and mmap offset are (at the moment) the same.  And get rid of an extra
drm_gem_free_mmap_offset() call (since it is already called from
drm_gem_object_release())

Signed-off-by: Rob Clark <robdclark@gmail.com>