fbdev: Prepare generic architecture helpers

Generic implementations of fb_pgprotect() and fb_is_primary_device()
have been in the source code for a long time. Prepare the header file
to make use of them.

Improve the code by using an inline function for fb_pgprotect()
and by removing include statements. The default mode set by
fb_pgprotect() is now writecombine, which is what most platforms
want.

Symbols are protected by preprocessor guards. Architectures that
provide a symbol need to define a preprocessor token of the same
name and value. Otherwise the header file will provide a generic
implementation. This pattern has been taken from <asm/io.h>.

v3:
	* include the correct header files
v2:
	*  use writecombine mappings by default (Arnd)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417125651.25126-2-tzimmermann@suse.de

include/asm-generic/fb.h

 /* SPDX-License-Identifier: GPL-2.0 */
+
 #ifndef __ASM_GENERIC_FB_H_
 #define __ASM_GENERIC_FB_H_
-#include <linux/fb.h>
 
-#define fb_pgprotect(...) do {} while (0)
+/*
+ * Only include this header file from your architecture's <asm/fb.h>.
+ */
+
+#include <linux/mm_types.h>
+#include <linux/pgtable.h>
+
+struct fb_info;
+struct file;
+
+#ifndef fb_pgprotect
+#define fb_pgprotect fb_pgprotect
+static inline void fb_pgprotect(struct file *file, struct vm_area_struct *vma,
+				unsigned long off)
+{
+	vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
+}
+#endif
 
+#ifndef fb_is_primary_device
+#define fb_is_primary_device fb_is_primary_device
 static inline int fb_is_primary_device(struct fb_info *info)
 {
 	return 0;
 }
+#endif
 
 #endif /* __ASM_GENERIC_FB_H_ */