drm: sti: remove useless call to dev->struct_mutex

No need to protect debugfs functions with dev->struct_mutex

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464630800-30786-19-git-send-email-daniel.vetter@ffwll.ch