[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.

Fixes: 1b06fcf5 ("[media] media: mtk-mdp: fix build error")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>