drm/ast: Gen7: Disable VGASR0[1] as on Gen4+

Set VGACRB6[5], which disables asynchronous sequencer resets via
VGASR0[1]. This was most likely an oversight when adding support
for Gen7. Aligns Gen7 with the earlier Gen4+.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-10-tzimmermann@suse.de