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