drm/ast: Gen7: Switch default registers to gen4+ state

Change the default register settings for Gen7 to mach Gen4 and
later. Gen7 currently uses the settings for Gen1, which is most
likely incorrect.

Using Gen4+ settings enables E2M linear-access modes in VGACRA2.
It appears to be related to the chip's PCIE2MBOX feature, which
is unused.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-11-tzimmermann@suse.de