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

drivers/gpu/drm/stm/ltdc.c: In function 'ltdc_plane_atomic_check':
drivers/gpu/drm/stm/ltdc.c:694:13: warning:
 variable 'src_y' set but not used [-Wunused-but-set-variable]
  u32 src_x, src_y, src_w, src_h;
             ^
             ^
drivers/gpu/drm/stm/ltdc.c:694:6: warning:
 variable 'src_x' set but not used [-Wunused-but-set-variable]
  u32 src_x, src_y, src_w, src_h;
      ^
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1538131180-34108-1-git-send-email-yuehaibing@huawei.com