drm/sti: remove set but not used variable 'priv'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/sti/sti_crtc.c: In function 'sti_crtc_vblank_cb':
drivers/gpu/drm/sti/sti_crtc.c:255:22: warning:
variable 'priv' set but not used [-Wunused-but-set-variable]

It never used since introduction in
commit 9e1f05b28009 ("drm/sti: rename files and functions")

Signed-off-by: YueHaibing <[e-mail]>
Signed-off-by: Benjamin Gaignard <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/sti/sti_crtc.c

···
 	struct sti_compositor *compo;
 	struct drm_crtc *crtc = data;
 	struct sti_mixer *mixer;
-	struct sti_private *priv;
 	unsigned int pipe;
 
-	priv = crtc->dev->dev_private;
 	pipe = drm_crtc_index(crtc);
 	compo = container_of(nb, struct sti_compositor, vtg_vblank_nb[pipe]);
 	mixer = compo->mixer[pipe];