fbdev: allow apertures == NULL in remove_conflicting_framebuffers()

Interpret (otherwise-invalid) NULL apertures argument to mean all-memory
range. This will allow to remove several duplicates of this code
from drivers in following patches.
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/b5fed539fe569035139c8594c23effd8c39f12ca.1535810304.git.mirq-linux@rere.qmqm.pl

drivers/video/fbdev/core/fbmem.c

@@ -1797,11 +1797,25 @@ int remove_conflicting_framebuffers(struct apertures_struct *a,
 				    const char *name, bool primary)
 {
 	int ret;
+	bool do_free = false;
+
+	if (!a) {
+		a = alloc_apertures(1);
+		if (!a)
+			return -ENOMEM;
+
+		a->ranges[0].base = 0;
+		a->ranges[0].size = ~0;
+		do_free = true;
+	}
 
 	mutex_lock(&registration_lock);
 	ret = do_remove_conflicting_framebuffers(a, name, primary);
 	mutex_unlock(&registration_lock);
 
+	if (do_free)
+		kfree(a);
+
 	return ret;
 }
 EXPORT_SYMBOL(remove_conflicting_framebuffers);