drm/ast: vga: Clear EDID if no display is connected

Do not keep the obsolete EDID around after unplugging the display
from the connector.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 2a2391f8 ("drm/ast: vga: Transparently handle BMC support")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241015065113.11790-3-tzimmermann@suse.de