drm/qxl: simplify slot management

Drop pointless indirection, remove the mem_slots array and index
variables, drop dynamic allocation.  Store memslots in qxl_device
instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-4-kraxel@redhat.com