drm/exynos: Use devm_clk_get in exynos_mixer.c

devm_clk_get is device managed and makes error handling and exit code
simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>