qxl: drop mode_info.modes & related code.

very old qxl hardware revisions (predating qxl ksm support by a few
years) supported a fixed list of video modes only.  The list is still
provided by the virtual hardware, for backward compatibility reasons.

The qxl kms driver never ever looks at it, except for dumping it to
the kernel log at load time in case debug logging is enabled.  Drop
that pointless code.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1488363154-6889-2-git-send-email-kraxel@redhat.com