arch/loongarch: Implement <asm/fb.h> with generic helpers

Replace the architecture's fbdev helpers with the generic
ones from <asm-generic/fb.h>. No functional changes.

v2:
	* use default implementation for fb_pgprotect() (Arnd)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417125651.25126-7-tzimmermann@suse.de

arch/loongarch/include/asm/fb.h

@@ -5,19 +5,6 @@
 #ifndef _ASM_FB_H_
 #define _ASM_FB_H_
 
-#include <linux/fb.h>
-#include <linux/fs.h>
-#include <asm/page.h>
-
-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);
-}
-
-static inline int fb_is_primary_device(struct fb_info *info)
-{
-	return 0;
-}
+#include <asm-generic/fb.h>
 
 #endif /* _ASM_FB_H_ */