drm/qxl: drop unused fields from struct qxl_device

slot_id_bits and slot_gen_bits can be read directly from qxlrom instead.
va_slot_mask is never used anywhere.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-6-kraxel@redhat.com