drm/panthor: Fix undefined panthor_device_suspend/resume symbol issue

drm/panthor: Fix undefined panthor_device_suspend/resume symbol issue

panthor_device_resume/suspend() are only compiled when CONFIG_PM is
enabled but panthro_drv.c doesn't use the pm_ptr() macro to conditionally
discard resume/suspend assignments, which causes undefined symbol
errors at link time when !PM.

We could fix that by using pm_ptr(), but supporting the !PM case makes
little sense (the whole point of these embedded GPUs is to be low power,
so proper PM is a basic requirement in that case). So let's just enforce
the presence of CONFIG_PM with a Kconfig dependency instead.

If someone needs to relax this dependency, it can be done in a follow-up.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403031944.EOimQ8WK-lkp@intel.com/
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240304090812.3941084-4-boris.brezillon@collabora.com