drm/exynos/mixer: Remove unused local variable priv

Remove local variable 'priv' to fix GCC warning:

    drivers/gpu/drm/exynos/exynos_mixer.c: In function 'mixer_initialize':
    drivers/gpu/drm/exynos/exynos_mixer.c:840:29: warning: variable 'priv' set but not used [-Wunused-but-set-variable]
Signed-off-by: NKrzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: NInki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_mixer.c

@@ -837,8 +837,6 @@ static int mixer_initialize(struct mixer_context *mixer_ctx,
 			struct drm_device *drm_dev)
 {
 	int ret;
-	struct exynos_drm_private *priv;
-	priv = drm_dev->dev_private;
 
 	mixer_ctx->drm_dev = drm_dev;