drm/ast: Hide Gens 1 to 3 TX detection in branch

Gen7 only supports ASTDP. Gens 4 to 6 support various TX chips,
except ASTDP. These boards detect the TX chips by reading the SoC
scratch register as VGACRD1.

Gens 1 to 3 only support SIL164. These boards read the DVO bit from
VGACRA3. Hence move this test behind a branch, so that it does not
run on later generations.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117103450.28692-6-tzimmermann@suse.de