arch/x86: Include <asm/fb.h> in fbdev source file

Move the implementation of fb_pgprotect() to fbdev.c and include
<asm/fb.h>. Fixes the following warning:

  ../arch/x86/video/fbdev.c:14:5: warning: no previous prototype for 'fb_is_primary_device' [-Wmissing-prototypes]
     14 | int fb_is_primary_device(struct fb_info *info)
        |     ^~~~~~~~~~~~~~~~~~~~

Just including <asm/fb.h> results in a number of built-in errors
about undefined function. Moving fb_pgprotect() to the source file
avoids the required include statements in the header. The function
is only called occasionally from fb_mmap(), [1] so having it as static
inline had no benefit.

While at it, fix the codying style in fbdev.c.

Link: https://elixir.bootlin.com/linux/v6.3-rc7/source/drivers/video/fbdev/core/fbmem.c#L1404

 # 1
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230424084751.14641-1-tzimmermann@suse.de