powerpc/fb: Call internal __phys_mem_access_prot() in fbdev code

Call __phys_mem_access_prot() from the fbdev mmap helper
pgprot_framebuffer(). Allows to avoid the file argument of NULL.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230922080636.26762-6-tzimmermann@suse.de