fbdev/cirrusfb: 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 <[e-mail]>
Acked-by: Javier Martinez Canillas <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/video/fbdev/Kconfig

···
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT
+	select FB_IOMEM_FOPS
 	select VIDEO_NOMODESET
 	help
 	  This enables support for Cirrus Logic GD542x/543x based boards on

drivers/video/fbdev/cirrusfb.c

···
 	.owner		= THIS_MODULE,
 	.fb_open	= cirrusfb_open,
 	.fb_release	= cirrusfb_release,
+	__FB_DEFAULT_IOMEM_OPS_RDWR,
 	.fb_setcolreg	= cirrusfb_setcolreg,
 	.fb_check_var	= cirrusfb_check_var,
 	.fb_set_par	= cirrusfb_set_par,
···
 	.fb_copyarea	= cirrusfb_copyarea,
 	.fb_sync	= cirrusfb_sync,
 	.fb_imageblit	= cirrusfb_imageblit,
+	__FB_DEFAULT_IOMEM_OPS_MMAP,
 };
 
 static int cirrusfb_set_fbinfo(struct fb_info *info)