drm/exynos: fimc: Clock control is unused if !PM

Protect the fimc_clk_ctrl() function with an #ifdef CONFIG_PM guard to
avoid "defined but not used" warnings.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>