drm/sun4i: mark PM functions as __maybe_unused

drm/sun4i: mark PM functions as __maybe_unused

Disabling CONFIG_PM produces a compile time warning when these
functions are not referenced:

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c:1072:12: error: 'sun6i_dsi_runtime_suspend' defined but not used [-Werror=unused-function]
 static int sun6i_dsi_runtime_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c:1043:12: error: 'sun6i_dsi_runtime_resume' defined but not used [-Werror=unused-function]
 static int sun6i_dsi_runtime_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 133add5b5ad4 ("drm/sun4i: Add Allwinner A31 MIPI-DSI controller support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525155030.3667352-1-arnd@arndb.de