drm: exynos: mark pm functions as __maybe_unused

The rework of the exynos DRM clock handling introduced
warnings for configurations that have CONFIG_PM disabled:

drivers/gpu/drm/exynos/exynos_hdmi.c:736:13: error: 'hdmi_clk_disable_gates' defined but not used [-Werror=unused-function]
 static void hdmi_clk_disable_gates(struct hdmi_context *hdata)
             ^~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/exynos/exynos_hdmi.c:717:12: error: 'hdmi_clk_enable_gates' defined but not used [-Werror=unused-function]
 static int hdmi_clk_enable_gates(struct hdmi_context *hdata)

The problem is that the PM functions themselves are inside of
an #ifdef, but some functions they call are not.

This patch removes the #ifdef and instead marks the PM functions
as __maybe_unused, which is a more reliable way to get it right.

Link: https://patchwork.kernel.org/patch/8436281/
Fixes: 9be7e989

 ("drm/exynos/hdmi: clock code re-factoring")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>