drm/msm/mdp5: mark runtime_pm functions as __maybe_unused

drm/msm/mdp5: mark runtime_pm functions as __maybe_unused

When CONFIG_PM is disabled, we get harmless warnings about unused
functions:

drivers/gpu/drm/msm/mdp/mdp5/mdp5_kms.c:1025:12: error: 'mdp5_runtime_resume' defined but not used [-Werror=unused-function]
 static int mdp5_runtime_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/msm/mdp/mdp5/mdp5_kms.c:1015:12: error: 'mdp5_runtime_suspend' defined but not used [-Werror=unused-function]
 static int mdp5_runtime_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~

This marks both functions as __maybe_unused so the compiler
can drop them silently.

Fixes: d68fe15b1878 ("drm/msm/mdp5: Use runtime PM get/put API instead of toggling clocks")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Clark <robdclark@gmail.com>