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

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

This macro allows 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: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221129191942.138244-9-paul@crapouillou.net