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: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>