drm/msm: mark PM functions as __maybe_unused

The suspend/resume functions are not referenced when power
management is disabled:

drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c:1288:12: error: 'dpu_runtime_resume' defined but not used [-Werror=unused-function]
drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c:1261:12: error: 'dpu_runtime_suspend' defined but not used [-Werror=unused-function]

This marks them as __maybe_unused to let the compiler
drop the functions without complaining.

Fixes: 591225291ca2 ("drm/msm: Add SDM845 DPU support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Clark <robdclark@gmail.com>