drm: gm12u320: Remove #ifdef guards for PM related functions

Use the pm_ptr() macro to handle the .suspend / .resume / .reset_resume
callbacks.

This macro allows the suspend and resume functions to be automatically
dropped by the compiler when CONFIG_PM 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. It also allows to drop the
__maybe_unused tags.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221129191942.138244-11-paul@crapouillou.net