drm/ast: Split ast_detect_tx_chip() per chip generation

drm/ast: Split ast_detect_tx_chip() per chip generation

Gen4 and later models detect the TX chip from VGACRD1, while earlier
models detect from VGACRA3. Split up the detection helper into
two separate helpers. Use SZ_ constants instead of plain numbers.

Then inline the call into its only caller ast_device_create(). When
ast_device_create() gets split up per Gen, either call will remain.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250922083708.45564-4-tzimmermann@suse.de