fbcon: Move fbcon callbacks into struct fbcon_bitops

fbcon: Move fbcon callbacks into struct fbcon_bitops

Depending on rotation settings, fbcon sets different callback
functions in struct fbcon_par from within fbcon_set_bitops(). Declare
the callback functions in the new type struct fbcon_bitops. Then
only replace the single bitops pointer in struct fbcon_par.

Keeping callbacks in constant instances of struct fbcon_bitops
makes it harder to exploit the callbacks. Also makes the code slightly
easier to maintain.

For tile-based consoles, there's a separate instance of the bitops
structure.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/20250909124616.143365-5-tzimmermann@suse.de