drm/gma500: fix double freeing

We are allocating backing using psbfb_alloc() and so
backing->stolen is always true. So we were freeing backing two times.
Moreover if we follow the execution path then we should be freeing
backing after we have released the helper. So remove the one which frees
backing before the helper is released.
While at it the error labels are also renamed to give a meaningful
name.

[Patrik: Fixed conflict with removal of struct_mutex]

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>