drm/ast: Move Gen1 device initialization into separate helper

drm/ast: Move Gen1 device initialization into separate helper

Split off device initialization for Gen1 hardware into the helper
ast_2000_device_create(). The new function is a duplicate of their
counterpart in ast_main.c, but stripped from most non-Gen1 support.

Simplifies maintenance as the driver's number of supported hardware
generations grows.

v2:
- remove unnecessary widescreen-detection logic

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250922083708.45564-6-tzimmermann@suse.de