drm/stm: dsi: use new SYSTEM_SLEEP_PM_OPS() macro

Use RUNTIME_PM_OPS() instead of the old SET_SYSTEM_SLEEP_PM_OPS().
This means we don't need  __maybe_unused on the functions.

Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Acked-by: Yannick Fertre <yannick.fertre@foss.st.com>
Tested-by: Yannick Fertre <yannick.fertre@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240129104106.43141-2-raphael.gallais-pou@foss.st.com