fbdev/vesafb: Replace references to global screen_info by local pointer

Get the global screen_info's address once and access the data via
this pointer. Limits the use of global state.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231206135153.2599-4-tzimmermann@suse.de