fbdev/deferred-io: Move pageref setup into separate helper

fbdev/deferred-io: Move pageref setup into separate helper

Set up struct fb_deferred_io_pageref in the new helper function
fb_deferred_io_pageref_lookup(), which runs when the pageref is first
taken. Remove the setup code from the rest of the code.

At first, the code allocates the memory of all pageref structs. The
setup of the various fields happens when the pageref is required.

v2:
- fix typo in commit message (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-3-tzimmermann@suse.de