drm/msm: Remove msm_gem_free_work

Now that we don't need struct_mutex in the free path, we can get rid of
the asynchronous free altogether.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>