fbdev/dnfb: Initialize fb_ops to fbdev I/O-memory helpers

Initialize the instance of struct fb_ops with fbdev initializer
macros for framebuffers in I/O address space. Set the read/write,
draw and mmap callbacks to the correct implementation and avoid
implicit defaults. Also select the necessary I/O helpers in Kconfig.

Fbdev drivers sometimes rely on the callbacks being NULL for a
default implementation to be invoked; hence requiring the I/O
helpers to be built in any case. Setting all callbacks in all
drivers explicitly will allow to make the I/O helpers optional.
This benefits systems that do not use these functions.

No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230927074722.6197-14-tzimmermann@suse.de

drivers/video/fbdev/Kconfig

@@ -217,6 +217,7 @@ config FB_APOLLO
 	default y
 	select FB_CFB_FILLRECT
 	select FB_CFB_IMAGEBLIT
+	select FB_IOMEM_FOPS
 
 config FB_Q40
 	bool

drivers/video/fbdev/dnfb.c

@@ -110,10 +110,12 @@ static void dnfb_copyarea(struct fb_info *info, const struct fb_copyarea *area);
 
 static const struct fb_ops dn_fb_ops = {
 	.owner		= THIS_MODULE,
+	__FB_DEFAULT_IOMEM_OPS_RDWR,
 	.fb_blank	= dnfb_blank,
 	.fb_fillrect	= cfb_fillrect,
 	.fb_copyarea	= dnfb_copyarea,
 	.fb_imageblit	= cfb_imageblit,
+	__FB_DEFAULT_IOMEM_OPS_MMAP,
 };
 
 static const struct fb_var_screeninfo dnfb_var = {