drm/exynos: fix building without CONFIG_PM_SLEEP

The runtime PM operations use the suspend/resume functions
even when CONFIG_PM_SLEEP is not set, but this now fails
for the exynos DRM driver:

exynos_mixer.c:1289:61: error: 'exynos_mixer_resume' undeclared here (not in a function)
  SET_RUNTIME_PM_OPS(exynos_mixer_suspend, exynos_mixer_resume, NULL)

This removes the #ifdef and instead marks the functions as
__maybe_unused, which does the right thing in all cases and
also looks nicer.

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