drm/exynos: mark exynos_dp_crtc_clock_enable() static

We get 1 warning when building kernel with W=1:
drivers/gpu/drm/exynos/exynos_dp.c:46:5: warning: no previous prototype for 'exynos_dp_crtc_clock_enable' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
So this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>