drm: rcar-du: Remove #ifdef guards for PM related functions

drm: rcar-du: Remove #ifdef guards for PM related functions

Use the DEFINE_SIMPLE_DEV_PM_OPS() and pm_sleep_ptr() macros to handle
the .suspend/.resume callbacks.

These macros allow the suspend and resume functions to be automatically
dropped by the compiler when CONFIG_SUSPEND is disabled, without having
to use #ifdef guards.

This has the advantage of always compiling these functions in,
independently of any Kconfig option. Thanks to that, bugs and other
regressions are subsequently easier to catch.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221129191942.138244-4-paul@crapouillou.net