fbdev: s6e8ax0: avoid unused function warnings

The s6e8ax0 suspend/resume functions are hidden inside of an #ifdef
when CONFIG_PM is set to avoid unused function warnings, but they
call some other functions that nothing else calls, and we get warnings
about those:

drivers/video/fbdev/exynos/s6e8ax0.c:449:13: error: 's6e8ax0_sleep_in' defined but not used [-Werror=unused-function]
drivers/video/fbdev/exynos/s6e8ax0.c:485:13: error: 's6e8ax0_display_off' defined but not used [-Werror=unused-function]

This marks the PM functions as __maybe_unused so the compiler can
silently drop them when they are not referenced.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>