drm: exynos: make PM functions as __maybe_unused

The newly added runtime-pm support for exynos-drm encloses the
pm functions in an #ifdef, but not the functions that are called
from them and nowhere else, which produces warnings:

drm/exynos/exynos_drm_dsi.c:733:13: warning: 'exynos_dsi_disable_clock' defined but not used [-Wunused-function]
 static void exynos_dsi_disable_clock(struct exynos_dsi *dsi)
             ^
drm/exynos/exynos_drm_dsi.c:1291:13: warning: 'exynos_dsi_disable_irq' defined but not used [-Wunused-function]
 static void exynos_dsi_disable_irq(struct exynos_dsi *dsi)
             ^

This removes the #ifdef and instead marks the functions as
__maybe_unused, so gcc can silently discard them and all called
functions when CONFIG_PM is disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>