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 9e1f05b2

 ("drm/sti: rename files and functions")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1541818660-37168-1-git-send-email-yuehaibing@huawei.com