backlight: fix detection of multiple graphic cards

Follow-up for e0504dd011189d97a1ea813aabfe1e696742bcf5.

Hopefully, devices in PCI subsystem have some properties, thus have
their udev database file. But, that may not be true. Here, we only read
sysattrs of enumerated devices, hence it is not necessary to check if
the device is initialized or not.

(cherry picked from commit 9990552f9a26cfb07ef3edaad846543bdd013b10)

src/backlight/backlight.c

@@ -55,6 +55,10 @@ static int has_multiple_graphics_cards(void) {
         if (r < 0)
                 return r;
 
+        r = sd_device_enumerator_allow_uninitialized(e);
+        if (r < 0)
+                return r;
+
         r = sd_device_enumerator_add_match_subsystem(e, "pci", /* match = */ true);
         if (r < 0)
                 return r;