gpu: nvgpu: Swap order of free/rb_erase

gpu: nvgpu: Swap order of free/rb_erase

If rb_erase() is called after __balloc_do_free_fixed() then the
rb_tree code crashes when trying to dereference the possibly changed
(or poisoned in the case of debugging) data in the rb_node.

Change-Id: I4a4456a5ec453fd9ab117c804dc19b2c048a61d4
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: http://git-master/r/755646
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Krishna Reddy <vdumpa@nvidia.com>
Reviewed-by: Ian Stewart <istewart@nvidia.com>
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>