drm: proc: Use remove_proc_subtree()

Use remove_proc_subtree() rather than remove_proc_entry() to remove a
minor-specific drm proc directory and all its children.

Things could theoretically be improved by storing the drm_minor pointer in the
minor-specific dir proc_dir_entry struct data and then scrapping the list of
proc files - but that's shared with the debugfs interface where you can't do
that, so I don't see an easy way of doing it.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: dri-devel@lists.freedesktop.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>