drm/ast: Blank with VGACR17 sync enable, always clear VGACRB6 sync off

Blank the display by disabling sync pulses with VGACR17<7>. Unblank
by reenabling them. This VGA setting should be supported by all Aspeed
hardware.

Ast currently blanks via sync-off bits in VGACRB6. Not all BMCs handle
VGACRB6 correctly. After disabling sync during a reboot, some BMCs do
not reenable it after the soft reset. The display output remains dark.
When the display is off during boot, some BMCs set the sync-off bits in
VGACRB6, so the display remains dark. Observed with  Blackbird AST2500
BMCs. Clearing the sync-off bits unconditionally fixes these issues.

Also do not modify VGASR1's SD bit for blanking, as it only disables GPU
access to video memory.

v2:
- init vgacrb6 correctly (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: ce3d99c8 ("drm: Call drm_atomic_helper_shutdown() at shutdown time for misc drivers")
Tested-by: Nick Bowler <nbowler@draconx.ca>
Reported-by: Nick Bowler <nbowler@draconx.ca>
Closes: https://lore.kernel.org/dri-devel/wpwd7rit6t4mnu6kdqbtsnk5bhftgslio6e2jgkz6kgw6cuvvr@xbfswsczfqsi/


Cc: Douglas Anderson <dianders@chromium.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.7+
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20251014084743.18242-1-tzimmermann@suse.de