drm/gma500: Clear fbdev framebuffer with fb_memset_io()

The fbdev framebuffer is I/O memory, so clear it with fb_memset_io().
Fixes the following sparse warning:

../drivers/gpu/drm/gma500/fbdev.c:234:20: warning: incorrect type in argument 1 (different address spaces)
../drivers/gpu/drm/gma500/fbdev.c:234:20:    expected void *s
../drivers/gpu/drm/gma500/fbdev.c:234:20:    got char [noderef] __iomem *screen_base
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230522190637.7039-1-tzimmermann@suse.de

drivers/gpu/drm/gma500/fbdev.c

@@ -231,7 +231,7 @@ static int psb_fbdev_fb_probe(struct drm_fb_helper *fb_helper,
 	info->fix.mmio_start = pci_resource_start(pdev, 0);
 	info->fix.mmio_len = pci_resource_len(pdev, 0);
 
-	memset(info->screen_base, 0, info->screen_size);
+	fb_memset_io(info->screen_base, 0, info->screen_size);
 
 	/* Use default scratch pixmap (info->pixmap.flags = FB_PIXMAP_SYSTEM) */