fbdev: Avoid file argument in fb_pgprotect()

Only PowerPC's fb_pgprotect() needs the file argument, although
the implementation in either phys_mem_access_prot() or
pci_phys_mem_access_prot() does not use it. Pass NULL to the internal
helper in preparation of further updates. A later patch will remove
the file parameter from fb_pgprotect().

While at it, replace the shift operation with PHYS_PFN().

v5:
	* state function names in commit description (Javier)

Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230922080636.26762-2-tzimmermann@suse.de