drm/exynos: Use devm_clk_get in exynos_drm_fimd.c

devm_clk_get is device managed and makes error handling and exit code
simpler.
Also fixes an error related to returning 'ret' without initialising
with error code.

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>