drm/exynos: Fix typo in exynos_mixer.c

Correct spelling "sucessful" to "successful" in
drivers/gpu/drm/exynos/exynos_mixer.c

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>