drm/exynos: Make 'drm_hdmi_get_edid' static

Fixes the following warning:
drivers/gpu/drm/exynos/exynos_drm_hdmi.c:111:13: warning:
symbol 'drm_hdmi_get_edid' was not declared. Should it be static?

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