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

drivers/gpu/drm/panthor/Kconfig

@@ -6,6 +6,7 @@ config DRM_PANTHOR
 	depends on ARM || ARM64 || COMPILE_TEST
 	depends on !GENERIC_ATOMIC64  # for IOMMU_IO_PGTABLE_LPAE
 	depends on MMU
+	depends on PM
 	select DEVFREQ_GOV_SIMPLE_ONDEMAND
 	select DRM_EXEC
 	select DRM_GEM_SHMEM_HELPER

drivers/gpu/drm/panthor/panthor_device.c

@@ -402,7 +402,6 @@ int panthor_device_mmap_io(struct panthor_device *ptdev, struct vm_area_struct *
 	return 0;
 }
 
-#ifdef CONFIG_PM
 int panthor_device_resume(struct device *dev)
 {
 	struct panthor_device *ptdev = dev_get_drvdata(dev);
@@ -547,4 +546,3 @@ int panthor_device_suspend(struct device *dev)
 	mutex_unlock(&ptdev->pm.mmio_lock);
 	return ret;
 }
-#endif