drm/i915: make i915_drm_client_fdinfo() reference conditional again

The function is only defined if CONFIG_PROC_FS is enabled:

ld.lld: error: undefined symbol: i915_drm_client_fdinfo
>>> referenced by i915_driver.c
>>>               drivers/gpu/drm/i915/i915_driver.o:(i915_drm_driver) in archive vmlinux.a

Use the PTR_IF() helper to make the reference NULL otherwise.

Fixes: e894b724 ("drm/i915: Use the fdinfo helper")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230616093158.3568480-1-arnd@kernel.org

drivers/gpu/drm/i915/i915_driver.c

@@ -1803,7 +1803,7 @@ static const struct drm_driver i915_drm_driver = {
 	.open = i915_driver_open,
 	.lastclose = i915_driver_lastclose,
 	.postclose = i915_driver_postclose,
-	.show_fdinfo = i915_drm_client_fdinfo,
+	.show_fdinfo = PTR_IF(IS_ENABLED(CONFIG_PROC_FS), i915_drm_client_fdinfo),
 
 	.prime_handle_to_fd = drm_gem_prime_handle_to_fd,
 	.prime_fd_to_handle = drm_gem_prime_fd_to_handle,

drivers/gpu/drm/i915/i915_drm_client.h

@@ -47,8 +47,6 @@ static inline void i915_drm_client_put(struct i915_drm_client *client)
 
 struct i915_drm_client *i915_drm_client_alloc(void);
 
-#ifdef CONFIG_PROC_FS
 void i915_drm_client_fdinfo(struct drm_printer *p, struct drm_file *file);
-#endif
 
 #endif /* !__I915_DRM_CLIENT_H__ */