drm/sti: Remove drm_debugfs_remove_files() calls

drm_debugfs_cleanup() now removes all minor->debugfs_list entries
automatically, so it's not necessary to call
drm_debugfs_remove_files(). Additionally it uses
debugfs_remove_recursive() to clean up the debugfs files, so no need
for adding fake drm_info_node entries.

Cc: benjamin.gaignard@linaro.org
Cc: vincent.abriou@st.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Vincent Abriou <vincent.abriou@st.com>
Tested-by: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-14-noralf@tronnes.org