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 <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

···
 	return 0;
 }
 
-static int sun6i_dsi_runtime_resume(struct device *dev)
+static int __maybe_unused sun6i_dsi_runtime_resume(struct device *dev)
 {
 	struct sun6i_dsi *dsi = dev_get_drvdata(dev);
 
···
 	return 0;
 }
 
-static int sun6i_dsi_runtime_suspend(struct device *dev)
+static int __maybe_unused sun6i_dsi_runtime_suspend(struct device *dev)
 {
 	struct sun6i_dsi *dsi = dev_get_drvdata(dev);