[media] media: mtk-mdp: mark PM functions as __maybe_unused
A previous patch tried to fix a build error, but introduced another
warning:
drivers/media/platform/mtk-mdp/mtk_mdp_core.c:71:13: error: ‘mtk_mdp_clock_off’ defined but not used [-Werror=unused-function]
drivers/media/platform/mtk-mdp/mtk_mdp_core.c:62:13: error: ‘mtk_mdp_clock_on’ defined but not used [-Werror=unused-function]
This marks all the PM functions as __maybe_unused and removes
the #ifdef around them, as that will always do the right thing.