omapdrm: dss: mark runtime PM functions __maybe_unused

Using the SET_RUNTIME_PM_OPS() macro causes a warning about the
referenced functions when they are marked static but not __maybe_unused:

drivers/gpu/drm/omapdrm/dss/dss.c:1572:12: error: unused function 'dss_runtime_suspend' [-Werror,-Wunused-function]
drivers/gpu/drm/omapdrm/dss/dss.c:1584:12: error: unused function 'dss_runtime_resume' [-Werror,-Wunused-function]
drivers/gpu/drm/omapdrm/dss/dispc.c:4845:12: error: unused function 'dispc_runtime_suspend' [-Werror,-Wunused-function]
drivers/gpu/drm/omapdrm/dss/dispc.c:4860:12: error: unused function 'dispc_runtime_resume' [-Werror,-Wunused-function]

Fixes: b92f7ea5

 ("drm/omap: dss: Make use of the helper macro SET_RUNTIME_PM_OPS()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211205131612.3192652-1-arnd@kernel.org