drm/ast: Split ast_set_def_ext_reg() by chip generation

Duplicate ast_set_def_ext_reg() for individual chip generations
and move call it into per-chip source files. Remove the original
code. AST2100 and AST2500 reuse the function from earlier chips.
AST2600 appears to be incorrect as it uses an older function. Keep
this behavior for now.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-9-tzimmermann@suse.de