drm/ast: Move Gen6 device initialization into separate helper

drm/ast: Move Gen6 device initialization into separate helper

Split off device initialization for Gen6 hardware into the helpers
ast_2500_device_create() and ast_2500_detect_wide_screen(). The new
functions are duplicates of their counterparts in ast_main.c, but
stripped from most non-Gen6 support.

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

v2:
- simplify widescreen-detection logic

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250922083708.45564-11-tzimmermann@suse.de